RE Room plot cliffnotes ver.

Wesker in 5 was a flash clone. Accelerated aging and mental instability was worsened as (the clone's) body degraded and the virus could not sustain the clone the same way. Ultimately the cascading failure of the degrading clone, the virus, Wesker's preexisting psychological problems (genetic and otherwise) and constantly being hunted by Chris and the rest of the BSAA and other government agencies, resulted in the Kijuju incident as the stress only exacerbated the flash clone's flaws and shortcomings as it was already in the process of degrading. Wesker's subsequent "Death" (as far as the public is aware) was successful, however- He is undoubtedly alive, somewhere. Though it's unknown why he's faked his own death- the chances are it was in fact to get people off his back and he's still pulling the strings behind the scenes, and now with his pursuers off his trail it may be a feasible explanation for some of the more recent bioterror attacks on populated cities, with a few exceptions.