An interesting belief
that falls more under Gnosticism
than a separate religion
Cainists regard Cain, the son of Adam mentioned in the Bible/Torah/Qur'an, as a specially holy figure because he was the first murderer. Also, he killed his own brother, and had a mark placed on his head by God.
The notion was that by bringing murder into the world, Cain created an opportunity to renounce murder and assorted sins. You have to be a mystic yourself to fully understand this logic, I'm told.
There are or were some Cainites who went further (Of course you'll find some fanatics somewhere), who ended up glorifying murder and all other sins. The credo was that you should commit every sin possible in order to give yourself a really difficult redemption after repenting. Also, it gave God a chance to be infinitely generous when He forgives you. Think of it as hitting bottom in Fight Club.
Related ideas popped up in Tantric Buddhism around the same time and there's been some debate who influenced who first.
Most of my facts pulled from The Illuminatus! Trilogy. Great book, but dated 1975. Does Cainism exist still? Maybe, it's actually connected to some religions involving human sacrifice, such as the Aztecs.