Pre-Filmation had He-Man slaying the Glorm in the Ram Man mini-comic.
King Grayskull died while banishing Hordak in MYP.
Zodak's brother was eaten by King Hsss in MYP.
Evil Warriors slew Palace Guards in MVC's 200X comics.
We've had death in MOTU before. Mind you, not with any major characters. I do agree that these deaths seem to be shock value deaths, moreso than a well-written, natural outcome.