(Assuming you are talking about Milton's Paradise Lost)
Satan is evil - that is quite obvious in any work of literature, not only this one. However, here, he is such a smooth talker that we are led to believe his words, even though clearly we shouldn't. He is portrayed as such a dynamic character, with strong opinions and characteristics, as opposed to God, who is quite bland and boring here. Satan is describing God as a tyrant, a fascist despot who loves abusing his power - and from his perspective, this may very well be true. He was cast out of Heaven because he disobeyed the 'emperor,' and was made to live in Hell forever. Obviously his opinions about God are logical from this point of view because everything he is saying is true - if you disobey God's laws, you will suffer.
