* Quirinus Quirrell: He was possessed by Lord Voldemort and died when Voldemort was expelled from his body.
* The giant spider: Hagrid's pet Aragog died after being fatally wounded by the giant spider.
While not technically a character death, it is important to note that the Troll was killed by the giant spider.
It's also worth mentioning that Professor Dumbledore's phoenix, Fawkes, is believed to have been killed by the troll, but it was actually only injured and later recovered.