Actually I think the most gruesome death Trek has had was in a G rated Movie. The transporter deaths near the start of the movie as you hear the screams and see the outline of the bodies changing. Utterly gruesome, though an accident as opposed to homicide.
Oh yeah, forgot about that one. That WAS pretty unsettling. I know there was no PG-13 rating back then, but how on earth did that not earn the movie a PG, at least?