Why are Christian's scared of Heath Ledger's Joker?

Whats your thought's on Heath Ledgers permance on the Joker. Persnally i loved it! I love The Dark Knight best batman movie in my opion but i have a dozens of friends who are christians and they feel that Heath Ledgers performance on the Joker was a little to demonic and dark. He resbales what satan is. For me i just look at it as a unique character but christian's belive because he respesants anarchy and chaos hes to demonic just saying the joker they picture SATAN whats your thoughts and why do you think that? Why a fictional character can put the fear of God into Christian's heart