Board Thread:General Discussion/@comment-32883774-20200526064647

After the events of Alicization, due to what had happened to him, Kirito developed a philosophy that he began living by, as summed up in this monologue: "When people commit an act so heinous that they can never redeem themselves for it, there is only one place left for them to go: the grave, for they are at war with humanity itself and thus must be exterminated to prevent them from spreading their evil. Somebody's got to do it, the laws be damned. We cannot let these Oni run around and terrorize anymore."

In Japanese mythology, an Oni was born when a human commits an act so wicked that their appearance changes inside-out to reflect their twisted heart, and they can never go back to their former selves. Their transformation into an Oni reflects the moment they crossed the line. Oni are devoid of ningensei, the quality that makes a human a human, and because of it, they are fundamentally opposed to humanity. In the context of Kirito, humans turn into "Oni" when they commit an extremely malicious act such that they have casted away their humanity in favor of their depraved desires.

Kirito rejects the norms of society in favor of his own moral code, his own idea of justice, which is best summed us as this: "When someone crosses the line and becomes a Oni because of it, when you see one, exterminate them on sight. They are to be given a death that reflects their own wicked acts. That is true justice." Simply put, Kirito becomes a vigilante.

Kirito sets out to demonstrate his view of justice: by killing the Death Gun trio, for which he gives each one a death that serves as fitting for their misdeeds. Why he would do this:
 * Kirito executes Kyouji Shinkawa by gunning him down with a Black Star pistol in his cell. The Black Star was the so-called Death Gun, and Kyouji (the creator of the Death Gun plot) picked it to be that because he found out that Shino Asada killed the bank robber with it.
 * Kirito executes Shouichi Shinkawa by repeatedly stabbing him to death with an estoc, reflecting on his weapon of choice and playstyle.
 * Kirito executes Atsushi Kanamoto with a succinylcholine syringe, in reference to him having done that to Kirito months earlier.
 * Kirito has had enough of the Laughing Coffin's attempts on his life and his friends' lives (in other words The Dog Bites Back ). There was Atsushi's attempt on him, and then Vassago/PoH threatening to brutally murder him and Asuna after having been beaten by him convinced him to use each and every method whenever possible to make sure that people like him never come after anyone ever again, even if it causes their death.
 * Kirito did not regret leaving Vassago/PoH for dead after turning him into a tree in the Underworld. He's not gonna regret executing them if giving the chance to do so (and very likely he will due to the above).
 * If Atsushi didn't attack Kirito (or was arrested before he could do anything to Kirito) he'd leave those two (and Kyouji) alone, but the fact that he did so means he is now at war with them and will stop at nothing short of executing them.
 * Atsushi and Shouchi are very likely on death row given that they're unrepentant killers (and would continue to kill if they weren't in prison). This alone give Kirito the moral high ground to condemn them with, and the government will give him legal immunity in executing them because of that evidence. 