Is the Killing Curse a spell Dumbledore would allow himself to cast? I, uh, never actually read far enough to find out, but it seems like the kind of arbitrary moral restriction he might have.
I can't recall him ever casting it, but I wouldn't put it beyond him, especially if he (as the OP states) believes the other is pure evil and must be destroyed.
True, but if they are both using their full powers, Gandalf still far outclasses Dumbledore. In the books he hardly ever uses magic and he killed a Balrog. With his full power, he wins.
Although in my head canon inspired by FD, I think it was FD, Dumbledore is a fragment of Gandalf that was sent into another dimension when he was killed, embodying a fraction of his power.