And yes, you could argue it was murder, but by that definition, letting Tuvix live would've constituted a double murder. The whole point of the story was that there was no right answer, that it was an agonizing, impossible choice to have to make.
Hmm... it's been a long while since I watched it, but if I remember the ep correctly, letting Tuvix live would not
be a double murder. Tuvok and Neelix had already been "killed" in an accident... Tuvix's death was premeditated murder in order to "resurrect" the original two.