How is he being let off the hook? He killed himself. It's more about trying to understand what causes people to actually do this. Just saying he's a selfish prick serves nobody any good. Not really any way to 'punish' him at this point.
Plus PLENTY of other people who we'd consider mentally ill are institutionalized, so they aren't let off the hook. And plenty of crazy people are in no way pitied. Who pities the Mansons of the world, for example, besides their handful of cronies?
I agree there is a very fine line in what you're saying, but I think there's a general understanding of what humanity considers a sane person or not. Otherwise you could very easily go down a rabbit hole of some philosophical discussion about free will and nature etc etc.
But also, giving sympathy doesn't mean people can't or shouldn't be held accountable. If someone with mental issues commits a crime, something not horrible to make this example easy
, we could still feel bad for them but still throw them in jail.