I am guessing something of a predatory or bestial nature; where the punishment fits the crime.
It was definitely because MacFinn's ancestors harbored snakes.
Joking aside, I don't think that there was an atrocity or a great "crime" in the sense that we're thinking. It doesn't make an awful lot of sense to curse someone by turning them into a nearly unstoppable killing machine for three nights a month. Like, you wouldn't punish Ted Bundy by giving him body armor and a belt-fed machine gun with easy access to an orphanage, you know? Saint Patrick gave him extraordinary, uncontrollable power, and basically unleashed him on the world. If he was an awful person who needed to be punished for, say, murdering a village-ful of women and children, you wouldn't turn him into a new apex predator; you'd turn him into
prey.
I think that the curse was done because the alternative was worse. And the whole "and your line will never die out" was negotiated by MacFinn as payment for services rendered. There must have been major badness going on in Ireland at the time. The Romans pulled out of Britain in the early fifth century, and there was quite a bit of chaos in the region; chaos brings supernatural predators (see my thoughts on Hastings, Starborn, and the deaths of the Winter and Summer Ladies). I think there was something going on with the Tuatha, Firbolgs, and Fomor.