To be fair, every single one of those people died and came back exactly once, and most didn't even come back fully, as most were zombies of some nature.
* Onyxia: The version we fought at 60 and the one at 8 are the same encounter lorewise, and the next time we encounter her, she's a barely thinking zombie.
*Nefarian: We fought him in Blackwing Lair, and the next time we see him he is a zombie dragon, though a better quality one than his sister.
*Kael'thas was killed in The Eye, and the next time we see him, he's a demon-zombie thing.
*Anub'arak was killed in Azjol-Nerub, and is brought back as an ice-zombie spider.
*Gara'jal: We encounter him once in Mogu'Shan, with the warning we will see him again when his body dies. The next time we see him, he is a spirit (AKA still dead).
Hogger stands as the odd man (Gnoll) Out, because he's the only one on that list that's still alive the second time we run into him.