I think that both NCC and Tom are right. Janeway did kill Tuvix and of course murder is normally wrong ... but being all alone in the Delta Quadrant and in dire need of two crewmembers is anything but normal.
What I liked about the episodes is that it did not pretend that there is an easy or natural solution.
Sometimes folks who discuss it go down that road (in order to rationalize for whom they are siding) and claim e.g. that Tuvix is a freak, a kind of involuntary and innocent vampire who sucked two people into him and cannot be allowed to live on as he would be a constant reminder of the people he swallowed up. But it is not his fault, there was no agency involved, it was just an accident.
The other argument would be something like murder is always wrong, Tuvix is an intelligent lifeform and his rights may not be violated and so on. But what about the rights of the people who died when he was born and who can be resurrected when he dies?
Neither Tuvix' birth nor Tuxok's and Neelix death are natural. Of course they constitute an accident that happened in nature, it was some kind biological symbiosis with the flower or whatever, but this accident is not natural in the sense of normal or not supposed to be potentially interfered with.
I like stories in which people have to make radically contingent decisions, i.e. cannot rely on anything. There is no floor underneath Janeway, she is out there all on her own and whatever she does will be her act and her guilt until the end of her days.