B-b-both?!?!??!

Uh... Both?

Wait... Come to think of it, yeah Dumbledore might've already been dead but that long fall had to have cracked a skull...

Severus Snape, you're a monster...

How could he have survived the killing curse?

The Killing Curse obv. It kills instantly

The fall would have killed him, obviously, but he was dead as soon as he was hit by the killing curse.

The Astronomy Tower is stated to literally be the tallest tower in all of Hogwarts, if I remember correctly... so falling from it would certainly be fatal, but the same can be said of the Killing Curse- which does have the word 'killing' in it, and for good reason.

Didn't voldemort say in OOTP that ou have to mean it for ti to work. snape and dumbledore were friends so maybe he wouldnt have meant it only enough for him to fall and die