He has a fierce anger streak that is often brought about by righteous indignation. The priest had insulted his dead brother and had twisted his brother’s religious experiences, claiming that his visions were the exact opposite of holy, and instead of the devil.
Louis was still grieving his brother’s loss terribly and after his death had come to believe he had done his brother grievous harm by dismissing his visions. Then this priest comes along and practically pronounces his brother is in Hell where he belongs. Louis saw red, and even in his drained state, was overcome with anger on his brother’s behalf.
I think too, that Louis wasn’t just beating the priest in that scene, but was attacking the part of himself that he saw in the priest’s hypocrisy. He had rejected his brother, and hated himself for it, and the priest was a physical manifestation of that rejection.