He's a self-styled exorcist, a spiritual vigilante who's spent the past 19 years trying to smack Satan into submission.
...AAAAAAnd that is how you end up possessed by the devil. A mere pompous, proud human doesn't "smack satan into submission".
You need the power of God, clothed with Obedience and Humility, in the state of Grace (i.e., not owned by the devil already), to do battle with the devil.
Here is what happens when people take on the devil without being appointed by the Church to do so:
Acts of the Apostles chapter 19:
13 Now some also of the Jєωιѕн exorcists who went about, attempted to invoke over them that had evil spirits, the name of the Lord Jesus, saying: I conjure you by Jesus, whom Paul preacheth. 14 And there were certain men, seven sons of Sceva, a Jew, a chief priest, that did this 15 But the wicked spirit, answering, said to them: Jesus I know, and Paul I know; but who are you?
16 And the man in whom the wicked spirit was, leaping upon them, and mastering them both, prevailed against them, so that they fled out of that house naked and wounded.
Those over 18 can fill in the details of just how the possessed man "mastered" them, and how they were both naked AND wounded...
This is what happens when you mess with the devil! Leave him alone. Self-defense, yes. But we're not Ghostbusters! Leave the professional demon-fighting to PRIESTS who are appointed by the CHURCH, having the shield of OBEDIENCE and a plausible claim of HUMILITY since they didn't take on the task themselves.