Nothing is new under the sun.
You're probably looking for an extreme version of the <PERSON> movement (heresy), which provides all the justification which a serial killer could want.
Roughly speaking (and almost certainly a cartoonish oversimplification), Cathars believed the God of the old testament was evil, and had created a corrupt world. Everything material was created by him, so everything material is evil. Later, <PERSON> came along, and he is a separate, good god. Also, souls are angels held captive by our (corrupt) bodies.
In practice, this seems to have led to a mostly-generic puritan lifestyle with a few quirks. Mostly, you had clean-living communities who worked hard and took care of each other. Some of those quirks were offensive to the church (refusing all sacraments, because they were worldly and so corrupt), some were neutral (no meat, dairy, swearing oaths, or killing), some were laudable at the time (frequent fasting), and one is exactly what you're looking for: the consolamentum.
Cathari society can be divided into the Believers and the Perfects. Believers are just regular guys. They try to live right and all, but they don't have any extreme requirements on them. Perfects are people who are dedicated to purifying themselves. 72
They live a strict ascetic lifestyle; surrender all possessions to the community; preach, pray, and do charity work all day; etc. And when they die, their souls go free to become angels and hang out with <PERSON>. The actual process of becoming a Perfect was a fairly mundane ritual named "the consolamentum," which I read as roughly comparable to a modern baptism: a certain aount of ritual, but nothing really exciting. After that, you just had to maintain the ascetic lifestyle until you died to get the good ending.
Any optimizer reading this would immediately see the loophole: behave however you want all your life, get this ritual on your deathbed, starve for however long it took you to finish dying, be an angel forever. It's not clear that it actually went this way very often, but it IS clear that the contemporary propaganda against the Cathars described them that way. There are also some specific charges that Cathars who timed this poorly (received a deathbed consolamentum, then recovered) were smothered by their friends to prevent them from screwing up. Libel or truth, that's your serial killer tie in.
So, you have this large, established body of doctrine to draw from. You have a creepy not-quite-business-as-usual religious vibe (including dormant angels inside people!). You have the standard serial-killeresque belief that everything worldly is corrupted (though with an established, existing religious belief behind it). And you have a very plausible excuse for killing people.
Also, it was based in France, so you might have to take a field-trip there for research ;) 590