Further to the link. Tuvix is a character in star trek voyager. There is a transporter accident that ends up welding 2 other characters (Lieutenant Tuvok, Neelix) into 1 individual. The episode is spent trying to resolve this issue.
By the end, Captain Janeway is given a solution. They can reverse the process and recover Tuvok and Neelix. Unfortunately this will destroy Tuvix. Tuvix, meanwhile has developed on his own. He doesn’t want to die and makes that clear. Janeway has the dilemma. She can do nothing, and let Tuvix live, or kill him to bring Tuvok and Neelix back.
Basically, it’s the trolley problem. Do nothing, and 2 people die, or kill 1 yourself, to save them.