Dumbledore said that since Voldemort tried out for DADA teacher, they haven't been able to keep one for more than a year. He acknowledged that there IS a curse, and that the DADA teachers never stay on for more than a year.
So, why, if he knew this, would he allow Snape to become one?
Because he knew that Snape would be leaving at the end of the year. Something would cause Snape to leave... or to be driven out...
And what better than killing Dumbledore to do it?
I was just rereading a part of HBP and it came to me. What do you think?