The problem is that neither Neelix nor Tuvok were dead (since they were able to bring them back) while Tuvix was effectively killed by Janeway's action. At no point in the episode does anyone say that there is a deadline for the recovery of Tuvok and Neelix, they could have done more research to try to save the three of them. Plus Tuvix was better than Tuvok and Neelix at their jobs!! Even for the sake of the ship, this execution was inexcusable. They present a solution that's horrible but we're supposed to accept it because the writers say we have to!!!
At least in the case of Sim, they had extenuating circumstances: The fate of the Earth was at stake, there was a deadline for the cure, and Trip was (apparently) indispensable to the war effort. None of that here!!!