Some years ago... Probably ten... I was involved in a heated discussion of the sort you're only going to find on USENET. No, not Imperial Star Destroyer vs. Starship Enterprise... There's geek and then there's loser geek. I've always tried to remain the former. No, it was a discussion of the Star Trek: Voyager episode entitled Tuvix.
A brief summary. As a result of a transporter accident, Tuvok and Neelix are fused into a new entity which takes the name Tuvix. He possesses the memories of both but he is a new, distinct individual. He is perfectly healthy and can remain as he is without any difficulty. Unfortunately for him, Captain Janeway isn't willing to let him do that. When the Doctor discovers a way to reverse the process, Janeway decides to proceed even though the cost will be Tuvix's life. Tuvix is unwilling to die and so Janeway has him escorted by security to the sickbay where she performs the procedure herself since the doctor is unwilling to take Tuvix's life against his will. In the end, Neelix and Tuvok are restored and Tuvix is no more. Janeway is obviously disturbed by what she has chosen to do, but she has done it and doesn't really seem to regret it.
At the time, I was VERY disturbed by this episode and in watching it again (thank you Spike) I am no less disturbed. There is really no justification for what Janeway does. It isn't as if the new composite entity can't survive (sort of a reverse The Enemy Within) and it is never suggested that the survival of the ship depends on the restoration of Tuvok and Neelix. Basically, Janeway decides to kill one person in order to save two. In some ways, Tuvix is criticized for his unwillingness to die for this purpose. Essentially, he seems to be treated as a coward because he does not choose to die.
Watch this episode if you get the chance. Very disturbing.