/facepalm at thread
Look, OP, yes, burning rome to the ground and killing the pope is fun, but as people have pointed out, he comes back.
Your best course of action is to vassal that poor fool, or take rome but put him on a island like Crete and then vassal him. Letting him have rome is preferable, since the pope always spawns there.
Alternatively, kill the old man and take rome, then ally the pope. A homeless pope is a weak pope, and if you can vassal him then, all the better.
What im trying to say is there are better alternatives to simply constantly killing popes as the magically re-appear, if your going to storm italy, do it with a clear plan of exactly what your going to do with his holiness.
Giving the pope an alternative settlement has worked well, but be careful which land to give the man, he tends to go on a killing spree of doom if you relocate him anywhere rich and with borders, thus my island suggestion.
EDIT: PS: going after the pope at all might by an iffy idea. Once his holiness declares war,

proverbially hits the fan, and the entire catholic world will want you deader than a 80's sitcom.