Essentially what Janeway did is to kill someone who doesn't want to donate organs. But she murders him in order to give his organs to two other people who need it.
The reason she killed Tuvix was to get a tactical officer and a cook back. With Tuvix, she would have only have one individual that is a tactical officer, and in his spare time cooks.
So, after she murdered Tuvix, what if Neelix decided to leave the next day? Would she have stopped him from doing so?