Look, there will be Satanists. There's a cult for any and every supernatural entity, just as there's a sexual fetish for anything you can think of. Some are just a handful of people, others are globally prevalent.
It's not unusual for sexual and religious interests to coincide. Most religions, small or large, fetishise violence (look at the emblem of Christianity!), so it stands to reason that sexual violence will find common purpose with some elements of religiosity.
The fact that Satan is viewed as evil by some of the world's largest religions doesn't make it any worse than any other belief - but does make it more likely to attract people wanting to do things they consider evil. There will be some amongst them who combine an interest in sexual violence with Satanism - just as some members of other religions combine their similar interests.
The problem isn't the supposed supernatural entities being invoked by one group or another, it's the violence. Perpetrators simply use religious rituals to wrap their crimes in a veil of meaning, whether they claim to be beating evil spirits out of a child or imbuing choir boys with Christ's message.
In the Irish murder linked above, the victim's body was found with a silver cup and communion wafers. He was tied to the rafters, and I bet he was tied in a crucifixion pose. It's unlikely to have been a Satanist murder, although it doesn't matter which entity the perpetrator thought he was honouring. What matters is the crime.