So you have a character you're just dying to kill. Maybe he is a real villain and deserves to die. Or, instead, she is a sweet child and everybody cries when she slips away in the middle of the night. Regardless, there's a right place, time and reason to get rid of that character and it HAS to move the story forward.
This is a writing Master Class, with the dual benefits of digging deeply into the psychological reasons for killing a character, and of building a believable story structure around a death.