Slipknot gig offends Greek church

Promotes Satanism?
No.
You Don't say.
I don't think they do, their music might be about mutilation and hatred, but they seldomly mention God or the Devil, well not recently anyway. Pentagrams are predominantly pagan and protective in nature.
Are they Christian? I'm not sure what the band members beliefs are.
Well if you don't like it go away. The advantage of free will, is that you can make a choice, unless we have no free will of course, but that is a matter for the Theologians of course.
Don't go on Cutler, you'll offend someone.