You shouldn't be "shot down" for having an opinion. I would agree that it was a bold move, but only if there was nothing else involved in the motivation for that move. But I still think that Mischa's quitting gave them them the opportunity to use that death scene for high ratings and profit.
And as far as most people disliking her character, that wouldn't have been a good reason to kill her off. There have been many shows and soaps where the "hated" character actually carried the show and helped make it a success.
IE Dan on One Tree Hill. People hate him but I love him, he adds so many storylines. Hes like Julie Cooper but much more intense.
Anyway, Marissa may have been more dramatic in s3 (towards the end) but its only because they had to do that in order for her to be killed off (meeting volchock for ex). The writers knew what they were doing. If it wasnt for that, shed be like any other season. She carried that show.