The first thing we see in the film is Oskar pacing around his room, brandishing his knife and telling an imaginary victim to "squeal like a pig". "Ah," I said to myself when I first watched it, "the local psycho." I couldn't feel too negative about him because of the beautifully sad music playing as he places his hand on the window as if groping for a friend as Eli happens to arrive. As it quickly becomes apparent, Oskar is far more sympathetic than just "the local psycho" as I had first characterized him. He's a vulnerable little boy tormented by bullies. We see a bully his own size (Conny) intimidate him so much that he allows himself to be backed against the wall while being called "piggy". At this point, it became clear that this was the real "local psycho" and it eventually turned out he wasn't the only psychopathic bully around.
As a way of coping with his torment, he fantasizes about murdering his tormentors. He romanticizes murder to the point of keeping a scrapbook of newsclippings about serial killers. When Håkan starts killing children in the area, it isn't clear Oskar cares about the danger, and he just adds an article about it to his scrapbook. His pretending to stab Conny one night in the courtyard is what first attracts Eli's attention to him. As a vampire, the themes of death and violence surround Eli. It isn't a very good survival tactic to arouse a vampire's interest, but fortunately Eli's not hungry enough this night to eat this interesting person and ze lets him go. Eli later indicates that his desire to commit violence reminded Eli of hirself.
Oskar: "I don't kill people."Eli is a level higher on the food chain, and ze needs to eat. Deadly violence is a constant facet of hir life. One can imagine ze's always working on ways to kill people. Ze doesn't like the consequence of hir violence, though. After drinking a victim, ze cries over the corpse even after being a vampire for over 200 years. Ze mindmelds with Oskar to show him what it's like to be Eli, and in the book we see Eli's perspective as ze was victimized by an evil lord some 200 years prior. Yet, Eli doesn't think that violence not used for sustenance should always be avoided, as ze encourages Oskar to fight back against the bullies "harder than [he] dare[s]". Violence used for self-defense, not sadistically but simply to make enemies back off, is encouraged by Eli and the narrative.
Eli: "But you'd like to if you could. To get revenge, right?"
Eli: "I do it because I have to."
On a school field trip after Eli encourages him to hit back, Oskar grabs a stick and waits for the bullies to come after him. He doesn't just attack but instead waits until the bullies make their intent to use force on him known. He gives them a warning to back off, and they instead try to intimidate him into surrendering by playing on his weak self-esteem. Oskar finds strength in his relationship with Eli and hits back harder than he dares. As it turns out, Conny isn't particularly strong and just collapses in tears. The bullies frame Oskar as some mad aggressor, but he doesn't care because of the significance of asserting his own worth and making it clear that he's not to be messed with. I love the music that plays at this point, illustrating that reverence Oskar feels at achieving a sense of power over his own being. This is similar to Eli's violence as it is to assert his innate rights if not to literally feed himself. As the body of one of Eli's victims is discovered in the same area, at the same time, and with the same music, it links them in their aggression.
After Eli kills his best friend and his girlfriend, Lacke goes to Eli's apartment to kill the vampire. Oskar prepares to hurt him to save Eli, and he draws his knife. In shouting a warning, he wakes up Eli, who kills Lacke in front of his eyes. Despite his love for hir, he is horrified by this violent display. He drops the knife as if to reject violence from among his tools. His days romanticizing murder are over. When the grateful vampire emerges, hir face is covered in blood. Not even recognizing that Oskar might have a problem with the blood, ze lovingly kisses him. As much as it is a sweet romance, it also shows the way Oskar has flirted with violence through him physically demonstrating love for a being who represents a lifetime of murder. When Eli leaves his life, he discards his serial killer scrapbooks and tries to have a normal life.
Of course, the fates (i.e. author John Ajvide Lindqvist) aren't as kind to him. Jimmy, the psychopathic older brother of the bully Oskar injured, leads the other bullies to take revenge. They catch him in the pool and lock the teacher and the other students outside. Jimmy takes out a knife like the one Oskar rejected and tells him that he'll cut out one of his eyes unless he stays underwater for three minutes. Feeling helpless, Oskar accepts his death and goes along with Jimmy's plot. When it becomes clear Jimmy actually intends to kill him, the other bullies become anxious and one (Andreas) just breaks down in tears. Eli comes to rescue Oskar and kills the bullies actively participating in the attack, leaving Andreas to tell the tale. In the book, there are several other witnesses, and they describe Eli as an avenging angel. Eli does not kill to feed but rather in defense of another, sparing those not actively pursuing Oskar's death. This is perhaps the most noble form of killing, yet its details present moral ambiguity.
One of the local drunks asserts to his friends that "the death penalty has no justification in a society based on rule of law". In the book, Lacke also reads Dostevsky, who opposed the death penalty. Eli's killing of the bullies in effect carries out the death penalty because ze does not gain sustenance from their deaths. While I think ze should be able to kill them if necessary, I'm not convinced it is. The only person for whom it is probably necessary is Jimmy because he is the active aggressor and may be strong enough to be a threat. Vampires are strong, but Lacke--an average man--is able to knock Eli around. The other bullies, ze could probably just disable or scare away. Eli passes judgment on them and deems them worth killing for their actions. Oskar gains freedom because of this, and he joins Eli on the morally ambiguous road where the film implies he may end up another Renfield like Håkan.
Håkan is a pedophile, and Eli plays on his sexual attraction to hir to get him to serve hir. Though Håkan doesn't have much self-esteem, he crosses lines because of his commitment to expressing his sexuality. He repeatedly uses the services of child prostitutes despite wishing they would have sex with him out of love and not desire for cash. The idea that what he's doing is rape and is wrong doesn't hold any significance in his mindset. When he goes after a boy he intends to drain for Eli, he masturbates, giving his violence a vibe of sexual assault. This vibe becomes literal after Eli drains him but doesn't break his neck, and he comes back as a zombie-like vampire and tries to rape Eli. Fortunately, Oskar's friend Tommy is there to beat Håkan with a trophy and rescue hir.
In contrast to Eli's execution of the bullies, Tommy's attempt to kill Håkan is completely justified. Zombie!Håkan is an unthinking murderous rapist. He's dangerous and won't listen to reason. Even though he once could reason intelligently, the animalistic vampire infection has taken the wheel. Zombie!Håkan is analogous to a wild animal like a puma getting in your basement. Tommy kills him--or at least brutalizes him to a point where he can't move--when the situation utterly demands it to protect him and Eli, and this is thus a noble use of violence in my opinion. The book portrays it well, which is somewhat a given because Eli is one of the central protagonists.
Violence performed to protect others and only when there is no possibility of the aggressor backing down seems like a completely acceptable form of such. It is unusual, then, when Lacke becomes the antagonist for doing exactly this. Eli is a persistent threat to every human being. Fair or not, the most ethical path for a vampire is to never kill to eat because the good of the many outweighs the good of the few or the one, and thus either live a tortured existence or commit suicide. Virginia commits suicide via sunlight, but Eli never does, though ze does seriously consider it a couple times. Because ze values hir life over the lives of thousands of innocent human beings, this makes hir a pretty major problem.
Should Lacke kill Eli? Does he have that right? This goes back to the death penalty, but even that's not exactly relevant. The issue of the death penalty presented at the pub pertains specifically to laws enacted by the people. Lacke isn't some official executioner, and Eli was never tried as a murderer. By the laws set by society, he should go to the police and present evidence that Eli's guilty of this crime and let the government deal with it. Government is a system of force created to impose order on society, and it is generally considered moral when set up with a parliamentary system. Not everyone likes the government, though. Despite witnessing Eli's attack on Jocke, Göste declines to report it because he is afraid the police will suspect his involvement and interrogate him. In the book, Lacke narrates about how he intellectually agrees that the death penalty is a bad policy but feels that a man has an innate right to kill someone who murders someone close to him. This leads him to reject the laws of society and engage in vigilantism.
Now, vigilantism is very common in vampire fiction, mainly because the authorities don't recognize the existence of the supernatural. Lacke can be compared to the protagonist Jonathan Harker from Dracula, who goes after the titular count after he kills/vampifies Harker's wife and his wife's best friend. It is nothing but heroic when he slashes Dracula's throat and helps bring about his end. Likewise, Lacke is a man filled with conviction to kill the vampire who murdered his best friend Jocke and girlfriend Virginia. In LTROI, however, we are sympathetic to the vampire, so Lacke is cast as the antagonist. He is played as a bit of a jerk, disrespecting Oskar when the boy tries to intimidate him. Eli's subsequent devouring of him somewhat comes across as cathartic, but Oskar's horror gives the violence a disquieting tone, plunging the well-liked Eli back into moral ambiguity.
As Eli leaves him, Oskar tries to reject the violence represented by the vampire, but he comes to embrace it once more as Eli returns to save his life. He ultimately rejects the human world with its rules of ethics, joining Eli in hir ambiguous vampire world. In the short story sequel Let the Old Dreams Die, we see that this literally happens as Eli makes Oskar a vampire and they live happily ever after preying on humanity, though not having read it--it not yet being published in English--I can't comment fully on the story, only the tidbits referenced by the lucky Scandinavian fans who post on "We, the Infected".
So, what is LTROI actually saying? Best guess: Violence is always disturbing but is sometimes justified. It shouldn't be romaticized, nor done for sadistic pleasure, apathetic fulfillment of sexual desires, or as a form of revenge. Violence is acceptable for asserting your self-worth and making a statement about others not having the right to sadistically toy with you. Killing for sustenance is more ethical than killing for revenge, but it is still morally grey.