Interesting character development for Ogata. I believe he is truly a clinical psychopath, and the only thing in his life that has been gnawing away at him was not guilt or anything like that, but rather the realization that he's truly different (a psychopath), that there's no one else like him, who understands how he thinks and why he does what he does. He desperately doesn't want to be alone, hence his last-ditch effort to get Asirpa to kill him, which, if she did, would at least serve to reassure him in his last moments that there are other people like him in the world. This is why when Asirpa refuses to murder him, refuses to murder him even AFTER he's given her good reason, it ignites his suppressed rage and pain about being the only one not able to understand human empathy, and he draws his gun on her.
All that said, I think it would be most satisfying and fitting for him to die here. I think his character arc could end here and feel complete.