The wiki is here but basically a transporter incident merged Tuvok and Neelix. The resulting entity Tuvix claimed separating him back into the original crewmen was basically executing him. Janeway did it herself after the doc refused to kill Tuvix.

A freak transporter accident created Tuvix by joining Tuvok and Neelix into this dude, a person with distinct personality but somehow memories of both. The Status Quo, I mean Cpt. Janeway required that Tuvok and Neelix return, so he had to go back. Into the Transporter.

The abridged version:
Shitty transporter tech was casually hacked by a flower, so instead of tea the result was Tuvix (along with two dead crewmen of no significant importance).
It was obvious to everyone that such a horrible abomination absolutely cannot be allowed to live, but the brave captain decided against nuking it from orbit for the safety of the crew & just risked it for several days as it terrorised the ship, moving through corridors and even gaining access to the ships bridge.
It already began spewing some sort of plant pheromones to infect the crew info a false sense of actually liking the freak, but ultimately Janeway did what was necessary & axed the three begins apart (with a space axe, a modified transporter tech).
Much like in Red riding hood, inside the monster she found two survivors and the day was saved.
To celebrate Janeway ordered the daily coffee rations doubled for everyone (knowing fully well not much of the crew can stand that shit so she drank it all).

Janeway made it official Starfleet protocol to immediately execute any other such eldritch monsters sci-fi tech would have caused, but Voyager was too far from home base too upgrade their protocols as well.