Avada Kedavra
The Avada Kedavra spell kills people... Spelling might be wrong
Avada Kadava
He never killed a Dementor. He used a Patronus when Dementors were attcking him though. It's Expecto Patronum
Good people don't use killing spells. Instead, they stun people to move them fal off brooms or pillars and similar. Only the Death Eaters and Voldemort use the Avada Kedavara, or killing curse.
Voldemort's Killing Curse rebounded on him, Killing Voldemort.