Deadpool is basically the Marvel Universe version of a Looney Tunes cartoon character. He's annoying, he doesn't shut up, sometimes he pulls weapons out of nowhere and no matter how many times you kill him, he comes back to life.
His incessant healing factor is his power and it also causes his insanity, but it keeps the cancer that would otherwise kill him at bay. He is, to all intents and purposes, immortal - and this is a source of amusement in the current Messiah War storyline - X-Force has time travelled to the year 2974 to discover that Deadpool is still alive, having lived 500 years doing random crap before locking himself in a freezer for 400 years just because he saw Indiana Jones do it in a movie. Or something.