I assume you are speaking from a Christian perspective. Modern Satanism is very much akin to humanism, where the self is supreme, a form of self-worship. Satanists do not necessarily directly worship a god called Satan but revere themselves as gods. Some of a christian background may choose this 'religion' as it seems rebellious or 'scary'.
As for a genuine worship of Satan ... well at least Lucifer, there is a group of people in the middle-east called the Yezedis who revere Lucifer, they have a similar creation story to genesis but believe the 'peacock angel/feathered serpent' was a wise mentor to mankind, and that God is impersonal and distant from mankind. The Yezedis are very secretive about their religion and never refer to Lucifer as Satan, but call him the peacock angel or the feathered serpent. They don't drink blood or sacrifice children, but they worship Lucifer and think he is good rather than evil.
Also, in the past there were several pagan religions in the middle east that endorsed child-sacrifice, but these religions involved worship of various fertility gods rather than acknowledging a Judeo-Christian Satan. In the bible it is written that various kings endorsed such religions and this lead to the downfall of Israel and Judah. As answer to your question why would someone willingly worship Satan while knowing about Christianity, a need to be different, rebelliousness and perhaps a severe religious upbringing?