I find it quite interesting that Christianity (and consequently Islam) have very different ideas of who/what Satan is than their ancestor religion, Judaism. Traditionally, Satan was not an evil entity, or even an entity at all, but was rather the word used to describe the agent of God that tempted humanity. This is quite clear in the book of Job, where the satan (literally: 'the accuser', 'the adversary' or 'the opposer') is on friendly terms with God and is trying to tempt Job to abandon God.
Compare that with the imagery in Revelation, and modern Christian belief where the devil (or the beast) is the evil lord of the underworld who is at war with God. The two just don't add up.
Then there is the common misconception that the serpent in Genesis that tempts Eve is Satan. If that was the case it would be quite strange then for God to curse serpents to crawl on their bellies if it was actually the devil who did it. The word satan isn't even used in any of those passages, and to top it all off, the serpent didn't even come from Jewish mythology, but rather from the Epic of Gilgamesh!
It seems that despite what to me is overwhelming evidence even within Christian scriptures that Satan is a product of mythological evolution, many Christians still wish to believe that there is a malicious demoniac who is hellbent on ruining their lives. I cannot understand what the comfort in this belief could be. Perhaps it is just because they want to have someone to blame for all the things that go wrong in the world. Or perhaps it is just that they do not wish to let go of ideas that they grew up believing in, and do not like to admit they were wrong about something.