It's because the episode made it clear that Tuvix was a brand-new lifeform with a unique personality. He wasn't just Tuvok + Neelix, he was more than the sum of his parts. Restoring Tuvok and Neelix meant killing Tuvix, who was definitively established as a sentient, sapient, unique being who did not want to die.
Also, the way the episode portrays things makes it seem like Janeway killed Tuvix as some kind of punishment for "killing" Tuvok and Neelix when it was not in any way Tuvix's fault.
You know who should be on the list? Solok. He's a blatantly racist jerk who thinks Vulcans are better than everyone else. Oh, and he's a total hypocrite when it comes to emotional outbursts.
Most of the others on the list simply happen to be Designated Villains
. They aren't really bad people, they just happen to be on the opposite side of our favorite characters. The only reason they don't feel sympathetic is because as the audience, we know
that our "heroes" are right and these people are wrong. We have a hard time reconciling our knowledge of what's really going on with their actions, and so we tend to label them jerks, even though they really aren't.
Basically, they disagree with Captain Kirk, therefore they must be terrible people. That's how the logic goes...it's (obviously) inaccurate, though.