Ayoooo yeah that makes sense.
Tbh Mikoto is the nightmare scenario of succumbing to some of the worst impulses one can possibly have, and the results being, in a typical scenario, "Unforgivable." But we in MILGRAM know the context, and big companies not caring about their employees is the true villain here as usual, imho.
Trauma and Anger are not shown to be inherently dangerous. Bottling it up is what killed all those people and nearly killed Mikoto. I think that's entirely Yamanaka's point. Because if Mikoto had been repressing all his life, how was he supposed to know the danger he was approaching? Imo, Forgiving him is the ONLY possible way to save Mikoto and MAYBE those people (or at least their restless souls).
Because maybe if Mikoto knows there are those out there who know what's going on and can help him, he'll choose a different path before it gets that bad. Better yet, "John" can help manage the emotions better too, since the other parts had time in MILGRAM to Just Exist. That's me really banking on him waking up before his crime after MILGRAM though.
Even if that's not how it works, I sincerely believe that Mikoto was put on this path because Yamanaka wanted to show the true danger that repressing all your trauma and anger poses to all society. If Kazui's repression help show how it caused problems for individuals, Mikoto's predicament is about how repression can affect society at large.
This is hilariously ironic for Kayano "I'm going to contribute and be the best I can for everyone around me" Mikoto; he didn't choose that standard, he just fell prey to it in the worst way. I think 5 years in Mental Torture Prison and probably a lot of professional help will genuinely make him never do this again. Professional help is Not nonexistent in Japan, far from it, but Mikoto likely convinced himself over and over that he didn't need it, possibly didn't want to explain it to anyone out of fear/stigma or shame, and couldn't go anywhere anyway because work overtook his life.
All this to say that I believe his resentment for himself bled into the people he killed specifically because he felt they were making less of an effort, and were happier or more successful than him anyway. That undoubtedly would get to him if he saw it every day.
Furthermore, he would never have a reason to notice anything mentally wrong if he hadn't done this, because imo that's what'll trigger serious amnesia and more defined switiching: heavy stress and bloodshed. He likely would have kept going like that til he died. Maybe MILGRAM can give Mikoto's selves a chance to fix that; whether they can/will is up to them, but I forgive the crime because it's a demonstration of how mental health stigma and priorities areanged by profit margin can endanger Everyone. Forgiving isn't saying "it's ok that you did that," it's a chance at redeeming oneself. Or multiple selves.
Also, part of him (who is still Mikoto bc system responsibility and shared feeling and such) may have felt that they wanted to escape society without dying, so going to jail for life or being sentenced to death was preferable to killing himself. At least if he kills others, he looks scary and powerful, and puts the onus on someone else to punish him and take him away. THIS LIKELY WOULD NOT HAPPEN IN A REASONABLE ERA WHERE MENTAL HEALTH WAS PRIORITIZED AND NORMALIZED and in my heart of hearts i believe that's Yamanaka point, thanks for coming to my tedtalk lol