It stems from a very general human response to want those they don't like to die. Even if it means those they love die sooner.
Whenever this subject comes up, there always seem to be comments like: "We can't let this $group-of-bastards escape death!"
That, of course, ignores that $group-of-bastards is usually a small minority compared to the larger number of people who would benefit.
This very argument was made when our now fairly common transplant surgeries were first attempted.