From an Islamic standpoint,
You must first understand who God (whichever name you use) is. In the Abrahamics, He is the creator, the all powerful, the one who without we could not have come to be.
That being said, the Bible describes him as a fallen angel, one who fell from graces, but never really gets into why (please tell me if I'm wrong on that one)
The Quran goes on to describe what he did wrong, and why he is eternally our foe. He is not an angel, but rather another creation called Djinn who had raised himself up to the status as being among the angels. How he got there I can only speculate. When God Created man, he commanded the angels to bow to the man. all of the angels did (given they had no free will), but Satan CHOSE not to since he considered himself above man. This choice to disobey while knowing God and his power, and even acknowledging his authority, is why Satan has already been judged to the hellfire. But God allowed him to stay his punishment until the day of judgement with the rest of creation, and left Satan with little other than his whispers. Those Whispers are what has led to man's sins and detractment from God's will, even to the point that many now do not even believe he exists.
That is the simple version.