Janeway had no right to kill him against his will in this one. Tuvix was exactly right that it was an execution. It's the moral equivalent of killing Chakotay against his will and harvesting his organs to save the lives Kim and Paris who would otherwise die. Really it's worse, given Janeway doesn't even truely respect his life and sentience or is just clearly blinded by her favoritism towards Tuvok and Neelix. She even criticizes him for having Kes speak on his behalf? Yes begging not to be murdered by all means available is clearly a deplorable act.
Does that make Tuvix a coward for not sacrificing himself? Sure, it does. Is cowardice a capital crime? I don't think so. Who can really say with any amount of conviction that they'd sacrifice their life no question asked to save two others? You don't really know until you're in that situation I'd imagine.
Yeah, Janeway's hypocrisy in this one is self-evident. Especially given some of her future actions too. But for one in the past, as not to spoil things for you and because there are so many, doesn't this episode just seem funny when you compare it to Janeway's condescending attitude towards the Viidians in Phage? How she sternly lectures them for stealing Neelix's lungs, and that she's so much better than them for not killing that guy to save Neelix's life? Hypocrite.
Tuvix was a likeable character and the episode was beautifully done. But seriously, there's just no getting around the fact that Janeway murdered him. She can preach all about how it was to save two lives, but that's an excuse and doesn't make it right. She played favorites and played god and had no moral problem doing so. Even the freaking EMH refused to do it because it violated his ethical programing.