You can warn your players all you want, they don't seem to listen when you tell them they're acquiring a bad reputation. Then they get banned from the potter-I mean magic shop for life and complain about it. "It was only a little fireball" *whine whine* "We didn't burn down the *entire* town" "We saved their town, why are they upset about the collateral damage?". Or they wonder why enemies don't surrender when their bloodthirsty tendencies become known across the land. So watch your reputation. It could come back as something completely different than you'd expect.
Found this art on cracked.com but I'm sure it's around elsewhere as well