"good" "bad" Naming

One thing that really struck me is the theme of things/characters being good or bad. We have the "Good Angel", pretty understandable right? But then there is the "Evil Angel"? I thought angels where all suppose to be good. Then Methistophilis is called a bad devil but later someone calls  him a "good devil". Arent all devils suppose to be bad? I think that this is used to skew the lines of good and evil.