See this is exactly what I mean. The Kirk example is a grey area, that in real life would lead to way too many questions than could be answered in a single TV episode. It's an interesting thing to think about though, and gets into the whole nature vs nurture debate that we saw tackled a bit in episodes like the DS9 one with the baby Jem'Hadar. If you're ripped apart into two beings like that, are you essentially "born" good or evil and would you have the capacity for change?
Could each Kirk have gone on to become psychologically stable? Maybe, maybe not. Did merging them save one combined life that would've otherwise soon expired, or did it murder two individuals who could've gone on to have long happy lives? Lots of questions, ripe for debate. A grey area.
Whereas in the Tuvix example, there's nothing really grey about it. And regardless of which side you come down on in the Kirk debate, that's why I believe Janeway gets all the flak and Spock does not.