Risa

She should have simply teleported everyone on the ship into the Tuvix collective.

I think the thing that a lot of people end up missing out of the conversation is whether Janeway wanted to do it or not. The whole point of being Captain is making incredibly tough decisions for what's best for the entire crew. You can tell in the shot of her walking away that she is extremely upset by what she just had to do. The ordering of it was bad enough but then her effectively executing him herself? In the Janeway Memoir, Janeway even says that she's been haunted by the decision ever since she made it. Wondering whether or not what she did was right. It's not like she was skipping down to Sickbay to repeatedly shank Tuvix with a sharp rock that fell from a bridge console.

I think the reasoning is pretty solid, stemming from the many vs the one.

Keeping Tuvix saves 1 plant. Removing the plant and splitting Tuvok and Neelix back saves 2 lives.

2>1.

Is it still shitty? God damn right it is. But it's still logical, even if a bit sterile.