Nytmare
David Jose
But really if I can't motivate players with my plot hooks, killing their friends won't help any.
I'd imagine that when it happens at the bad end of the spectrum, it's usually less as a means of motivation and more just knee jerk trope usage.
I wouldn't want to venture a guess as to what the percentages are, but how many movies have you seen where if they bothered to write a character as having a parent or a child or a loved one or a pet, you can be almost certain that said loved one is going to be threatened with death or killed in the last act of the movie.