A nice colorful metaphor, but it doesn't change the facts. Regardless of how Tuvix came into being, he was alive, he was conscious and he was sentient. Janeway is captain, she's not god. She has no right to decide who lives and dies. Tuvix had as much right to be alive as anyone else on that ship. Janeway didn't respect that right and decided she'd rather have her friends back. She performed the procedure personally, so yeah.. she stuck that proverbial knife in him.