Also this question may sound a little stupid, but you know what I don't get? Why is it that some people believe in there's a god, and there's satan or devil whatever you call it? Why can't they just believe in god without thinking he has an enemy, is it because of that whole "You can't have good without bad" quote or what?
First off, just to clarify real quick, what I've found is that the quote "you can't have good without bad" is the opposite of the truth. In a way, in my opinion, all bad is simply perverted good. Having things isn't in itself bad, but stealing them from others is. Having sex within marriage isn't bad, but cheating on your spouse is. Killing to protect others isn't bad, but killing for some other unjust reason is. (Or perhaps killing is a perversion of living, I'm not really sure.) Looking at flowers isn't bad, but cheating off of someone's paper by looking at it is. Laughing isn't bad, but laughing because you caused someone pain is. Singing isn't bad, but singing excessively to the point where it annoys others is. Do you see what I'm saying?
Anyways, on to the whole Satan thing, in Christianity, Satan isn't really God's opposite, he's kind of, in my opinion, an example of something that perverted God's gift of freedom. In some ways, I think, he's a lot like what I believe people are, beings who have turned away from God because they want to do things their own way, perverting His gift of freedom. He is God's enemy, but he's wholly inferior, living off of borrowed time. God could destroy him if He wanted to, but He has given him the gift of freedom, so He doesn't destroy Him. Although He will punish him eternally at the end. Does that make sense? I'm sorry if it doesn't, I'm kinda' beat from school today.
I can't really say why other religions believe in Satan-like figures for sure and I would probably offend someone. Although I'm pretty sure a lot of them sort of mix good and bad into a bunch of Gods, like the Greeks, Romans, and Norse. But of course, then they have their own little twists which I do not know. I would say, because I'm a Christian and believe the Bible, that other religions that have Satan-like figures have them because they are vaguely aware of the truth of the Bible and have tried to imitate it. Once again, I hope that made sense. I also hope I didn't offend anyone. Feel free to ask questions as they help me to learn and to clarify for both you and myself.