He seemed so evil and mean even before he turned into the werewolf. Most werewolf movies the guy who turns into the wolf is a nice average guy. In this movie the priest seemed very evil and it wasnt explained. Ive never heard of a mean and evil priest before.
Jane & marty basically explain it in the film: "Maybe he (Reverend Lowe) is like this almost all the time. Only as the moon gets fuller..." Marty: .."the guy gets wolfier."
What others have mentioned elsewhere in the thread: He is gradually being taken over by his evil werewolf side, and his humanity (and conscience) are being destroyed.
"I'd say this cloud is Cumulo Nimbus." "Didn't he discover America?" "Penfold, shush."
^^ this. And anyway the priest didn't seem evil at all in the beginning. It was a progression. This movie isn't rocket science so I'm not sure any anyone would be confused.