We already know he wasn’t mentally stable, given the mix of Vulcan and Talaxian emotions within him. He could potentially use Tuvok’s emotional control skills, but we’ve seen in other episodes that those don’t necessarily work on non-Vulcans. Tuvix could have gone full crazy and tried to kill everyone, and could do it with Tuvok’s security codes.

What happens when his Vulcan side hits Pon Farr? With Neelix in the mix he might do worse than just kill everyone.