The vast majority of intelligent agents we know (they’re all people), if given a choice between killing everyone while feeling maximum bliss, or not killing everyone and living our regular non maximum bliss lives would choose the latter.
I'm not sure if I understand what makes this assumption so obvious. It seems intractable to actually know, since anyone who would choose the former would have every reason to lie if you asked them. It's also very easy to decieve yourself into thinking you'd do the thing that lets you feel better about yourself when it's all theoretical.
I like your post and it made me think a lot, I'm just confused about the niceness part. I feel myself being far more cynical of the extent of human niceness, but maybe that disagreement isn't important and you're just considering why that kind of behaviour might exist at all?
Personally I'd feel pretty confident that humans have probably caused far more suffering than they've caused pleasure/utility, regardless of whether we're talking about intentional vs incidental.
Consider that something like 1/3 (or more) of humans believe that an entity exemplifying infinite perfection permits the existence of a realm of torture for all eternity.
At any given time humans facilitate the excruciating existence of animals numbering an order of magnitude more than that of humans.
I think it's interesting to consider how those trends might correlate with rising numbers of people identifying as non-religious.
Regardless of whether changes in religion caused an increase in depression, I think it's certainly possible that it influenced how people might have felt writing about their life and experience.
Speaking as someone raised in a fundamentally religious setting, there's often a certain kind of guilt associated with expressing negativity about oneself or ones life. No matter how bad you feel about yourself, definitively calling yourself a "loser" would be an affront to the creator.
A lot of those typologies of cognitive distortions would be read as vain/worldly/unfaithful towards a divine plan. People might be experiencing all of that internally, but interpreting it as a spiritual failing to be expressed through spiritual language, if at all.