Is Avada Kedavra a bad word?
Avada Kedavra Obviously there’s no coming back from this spell, and according to the Ministry of Magic’s classifications (and, again, fake-Moody) this is the worst Unforgivable Curse.
Can you block Avada Kedavra?
There is no direct way to block the Avada Kedavra curse directly with a counter curse. One can dodge it or use magic to put something physical in its path. This battle scene is quite consistent with what Rowling laid down about the curse.