HALF-BLOOD PRINCE REVELATION (major spoiler alert !!!!)
I am rewatching it on Freeform and I realized--Snape killed Dumbledore because Dumbledore would have died anyway by the curse on Voldemort's ring. This would translate to Voldemort killing him, making Voldemort the master of the Elder Wand (of which Dumbledore was the current master).
Keeping the most powerful wand in existence from Voldemort is the most important thing, since Dumbledore is going to die anyway. Right?
Except...
Snape said he could contain the curse in Dumbledore's one hand for now, but it would spread, eventually killing him. So why not amputate the hand? Dumbledore's smart, he can make himself a fake one! Everyone wins.
Don't argue that because Dumbledore and Snape were wizards, they have never heard of amputation. Because we see Voldemort cutting off Wormtail's hand!
Siriusly. Wizards are dumb.
This is why Dumbledore was't a Ravenclaw.