Chances are snape didn't really want to kill Dumbledore.
Remember, in the begging Snape promised Narcissus (however you spell that) that he'd take care of Draco, no matter what!
When the lot were up in the tower he had to kill Dumbledore or Draco'd be in big troubleness. And he'd die too!
That was probably why he killed but most likely didn't want to