In Peter Parker: Spider-Man #28 (April 2001), our friendly neighborhood wall-crawler goes to great lengths to keep from killing Mendel Stromm even though the death seems like the only way to save the world and Mendel begs our hero to euthanize him. Why? Because Spider-Man doesn't kill, hasn't killed, won't kill, never will kill. Period.
Sorry to dissapoint you, but Spider-Man has killed at least once! Just take a look ate the Spider-Man versus Wolverine one-shot from 1987. Spidey is enraged and thinks Wolverine is about to attack him. But it was Charly, and she dies because of the force of the blow. She did commit suicide, in a way, but Spidey was the one to strike the fatal blow.
[Also see an earlier letter in To the Editor 01-Apr-2002 for info on Spider-Man's victim in Ultimate Marvel Team-Up #1 - Ed.]