Okay so I'm probably totally wrong but my theory about wtf is going on is something along these lines... timelines and time traveling.
In a certain timeline, the main character went into scientific research and decided to carry along his plan to kill everyone. That's why the hot chick is trying to build him up from an NPC into an MC, and that's why the memories from his past can tell what will happen next: those memories are shared by the two versions of the MC before the divergence in life paths.
Now, who is the hot girl? I'm not quite sure how it ties in, but I think its his wife. But young (I have a few theories about this but they don't quite add up)? I think that a big hint that its his wife is how she wears the wedding ring as if it were hers, and how at the beginning he mistakes her for his wife.
And she probably tried to make him into an MC a bunch of times but failed. And thats why there's so many brunette versions of her, and the only "good" version is the blonde one? I'm just brainstorming here, it's probably not right but it should be along those lines. Also the fact the brunette versions seem to lack a physical body and how evil MC would need to be in this timeline suggest there's like a merger of timelines or something. A more simple alternative would be that the bad guy is the fat guy.
This manga reminds me a lot to Shibatarian
https://mangadex.org/title/bfd18f9d-5441-43e5-8b37-1e83de3758c4/shibatarian but way way better and more seinen. In the sense that during their teen years a conversation between the MC and his friend the fantasy of killing everyone comes up, and now years later due to fantasy elements it comes into fruition.