The Devil is another name for Satan. A devil or a demon can mean a minor devil who rebelled with Lucifer and is condemned with Lucifer according to Christianity. Belief in the devil is in no way harmless. Countless innocent women have been burnt as witches because superstitious people believed they had sold their souls to the devil. This is just one example of the harm belief in the devil causes. Even in the 21st century some of the sillier Christian Fundamentalists subject helpless children or adults to traumatic Exorcism because they imagine someone is possessed by the devil.
Atheists feel that the existence of all supernatural beings are equally unlikely.
Satan in mythology
Satan is shown as a giant red fawn carrying a trident in today's media. This is because fawns were Greek spirits, and Christians denounced a great deal that was in other religions. The redness comes from Odin's red beard and the trident from Neptune. In older versions, he was blue to represent coldness. Neither of these are true, because he was described as being the most beautiful angel (although God might've uglified him when he rebelled).
In Islam Satan is a djinn.
Demons in Judaeo-Christian mythology are minor devils or fallen angels. The Bible teaches that a third of the angels rebelled with Lucifer and are damned in Hell with him. This is not a harmless fairytale. It is also believed that demons can possess Human beings. In the past a range of psychiatric illnesses were ascribed to demonic possession. Epilepsy and other types of brain damage could also be attributed to demonic possession as could anti social behaviour or atheism.
Patients who were already vulnerable would be told that the devil possessed them. This could easily aggravate any psychiatric illness. Bizarre and potentially traumatic rituals would be done to exorcise imagined demons. Sometimes psychiatric patients would be beaten to beat the devil out of them.
How is he worse then God?
In the Bible, God is shown to kill entire species, villages, and have people die and suffer. If Satan were to do the same, what difference does it make?
Obviously, Satan, like God, wants people to worship him. However, this shouldn't happen since he isn't a god, at least not according to Christians. So naturally, he decided to become one.
Satan has done terrible things, like torture Job, but as this was done for a bet with god it's not clear which of them was the most evil in this case.
Modern Christian fundamentalism
Satan in Popular Culture
Satan has appeared in many different form of media, like movies, comics and TV shows. Usually he is shown as the red fawn. In many of these, he is a comical character.