Before Skarr was a demon he was a man. After he was turned into Skarr he served Mad God Chaos. But he sought freedom from it. He used his son as a tool and made a bargain with him to serve him until his death. For a demon to gain freedom he needs to find soul blacker than his, so he made sure that his son's evil would surpass his.
Major Story Arcs
When Vampirella arrived he knew that she differs from regular vampires, and captured her. W. W. Wade thought he could get out of his deal with Skarr when he is a vampire and never dies of old age. He starved Vampirella until she drank from him. But Vampirella's bite doesn't carry the vampirism infection and Wade died, thus Skarr got his black soul he needed to buy his freedom from Chaos God.
Skarr has a contract with Pendragon, upon his death Skarr gets his soul, but until then he serves Pendragon.