It wasn't your fault. It was merely an unhappy coincidence that you happened to be the one standing behind the counter at 1:30am. It wasn't your fault that you were holding the knife, and it certainly wasn't your fault that the business end of the knife found its way into Nakaokaji's stomach. There was nothing you could have done to avoid it.
The police, for the most part, agreed. It was self defense, they ruled, and so you were released. The only one at fault was Nakaokaji himself.
Of course, this wasn't true at all. Nakaokaji was nearly as blameless as you. But the police had no way of knowing this, and no way to punish a dead man.
Funny, isn't it? No one can punish a corpse. In the end, all humans turn out the same.
Self defense or not, as you wash your hands for the fifth time in twenty minutes you can't forget that you killed a man.
You've always been a good upstanding Japanese citizen. Or maybe you haven't, maybe you've made your fair share of mistakes — no one is perfect, after all. But you've never been arrested. You've never been caught.
And, of course, ever since the news broke about Kira, you've been trying your very best to keep it that way. You follow the law to the letter. You discourage bad behavior in your peers. You show up to work five minutes early and remain focused and diligent till the moment your shift ends. Because you're a good person. You're not a criminal. Not like the late Nakaokaji.
You're sure Kira had nothing to do with Nakaokaji, of course. Kira only stops the hearts of the wicked. You are the only one who killed him. You are the only one with blood on your hands.
You're just as bad as he was, aren't you? He was a killer, and now you are too.
Self defense, you remind yourself, self defense. You were in danger. If you hadn't acted first he would have killed you. And you didn't mean to kill him. You just wanted to stop him. You wanted to save yourself. You didn't mean to kill him.
Does it matter that you didn't mean to kill him? The outcome is the same. It doesn't matter what you meant. A man still died at your hands. You're a killer.
And the grand killer, Kira, is watching.