As far as I recall, the only actual affects of the merging that we know for sure is that it removes impulse control and that it prevents death by tainting. I don't think being crazy is a requirement to accepting the merge, since I don't think Shinae was unusually immoral or crazy when she accepted--just desperate.
Snad's goal might have been somewhat just if she really wanted drow ruled by drow. But that's not really true--she wants them ruled by a specific drow--her. That might come from Diva, since Snad says Diva controlled every aspect of Snad's life.
Grow up in a controlling environment like that and you tend to become a control-freak too.
I also don't think the psychopathic behaviors started with the merge. I think they started with Khaless's death. Snad herself points to this as her breaking point
, and she wasn't tainted when it happened. Nor was she tainted when Kalki was born
. She chose to act because of that break, not the merge. But because the merge removes impulse control, it keeps her on her current path and might account for some of the sloppiness and rash behaviors that have been showing up lately.