I had to deal with this for my character. The DM's line was this:
If I EVER willingly changed into a werewolf, I was NPC'd.
Using the werewolf powers as written is an Evil act (according to my DM) and no evil PCs are allowed. However, I was unwilling to seek a magical cure. So...
In the end it worked out for me. For your cleric boy, if he murders unwillingly, I would say the cleric should still ask for forgiveness and possibly take up a quest for atonement. If he does it willingly, he's becoming Evil. Lawful Good gods should hold no truck with clerics that run around as CE beasts.