Howzer

The base-line isometric component to the VN is actually kind of base (but still fun), but Utawareramono is a combination of fantasy with a science-fiction twist at the end. It's not too long and would be an alright suggestion. Another one I ought to suggest that didn't already get suggested is Sekien no Inganock, which is steampunk.

Actually made an account to answer this with a small contribution: Kira Kira has a kind of breakdown route for Shiino where the MC's mental health takes a serious nose-dive. In the end of that route, he starts seeing things , but I think that's it for the entire VN, which may or may not fulfill your request. It definitely fulfills the "cracking under pressure idea" and I consider that route to be much better than the "true" route. Phenomeno could potentially go underneath this tagline too. It's a short, horror VN (that is actually creepy with great sound and ambiance...it actually delivers on creepiness) in which the MC does get carried away in the mystery. I can't really say much about it since it would spoil it, but it's short enough (at around 2 hours) that it wouldn't make too much of an effort to play. Getting it to work for me was difficult, so here's a link for the translation patch since I can't find it here on Fuwa: http://www.mediafire.com/download/caixmqj46hpp5lh/Phenomeno-Trial_en_patch-1.1.rar Other ones I would include would be Swan Song, but I can't think of anything at the top of my head outside of what Clephas said in which the MC does something that effectively dooms the world.