Lets take these in reverse order. I would like you to try to step out of your bias (i.e. as a member of a religion) and try to understand this rationally on a general sense.
5. In Greek mythology two of the characters were Zeus and Hecate. You probably don't believe in, or worship either. If someone in this religion came and told you you must worship Hecate because you didn't worship Zeus, you would tell them they were nuts.
People who don't believe in the Christian/Islamic narrative of God and Satan don't believe in either God or Satan. Whether they go to Hell (which they probably also don't believe in) is another story.
4) The idea that connection with the Devil causes mental health issues is probably unscientific.
People have been murdered in both Christian and Muslim context because they were accused of having such a connection (it seems to me that the murderers are the ones being evil).
People who don't accept the religious story won't have any connection to either of the characters.
3a) If you tell me that I am going to Hell, isn't it you who is being evil? Don't blame your hateful beliefs on me. Do you know that many Christians believe that all Muslims are going to Hell?
Unfortunately religious people seem to like standing around explaining telling each other they are going to Hell. I think this is a very ugly to do.
3b) Singing? Really? If either Christianity or Islam is correct, then I am going to Hell. Believe me, it is going to be for reasons that are a lot more enjoyable than singing.
2) In ancient religions, each little culture had their own God. Each little culture called all the other Gods evil. Their own God's magic was considered good (even when it involved murder or destruction).
Followers of the Christian/Islamic God have used quite a few spells that can only be called evil. God used magic to kill thousands of innocent babies in Pharonic Egypt. God then killed thousands of His own people in one night in the desert. There are even petty examples, the prophet Elisha magically summoned a bear to kill some kids who called him "baldy" (a little severe don't you think).
1) Satan is a mythological figure. Saying he is the purest of evil has no meaning outside of a specific religious context.
Real evil includes genocide and abuse and murder. All of these things are often done in the name of God. The crusades were done in the name of God by people wearing crosses. People were sold into slavery by people who followed God and wore crosses and crescents (not pentagrams).
Looking through history, it is obvious that compared to the great evil done by people claiming to worship God and wearing Godly symbols, the people claiming to worship Satan are pretty insignificant.