In the scriptures somewhere, the Buddha answers a few different people in a few different places about Kamma. They wanted to ask all sorts of deep questions about it. They were so deep; questions about "the beginning of time," "the end of time," "Where does a Buddha go after they die," and all sorts of far-reaching things. The Buddha simply said that if you try to figure out these questions with your own brain, you will go insane.
I can't tell anyone how a really messed-up person ever gets to the Dhamma. However I can give an example of somebody messed-up, yet they got to the Dhamma, but I can't tell you how it happened. For those of you who've read biographies of the Buddha, you'll know that one of his top disciple's was Mogallana. Mogallana was a brilliant human being, perfectly enlightened, and had super psychic powers, nearly equal to the Buddha. His wisdom wasn't quite equal, but he was still one of the most gifted people who ever walked this planet. However, in past lives, (going back hundreds), Mogallana was so messed up, he killed his mother and father. Because he killed his mother and father, his Karmic reward for that life was to go straight down to the bottom of hell. Somehow or other, he worked his way out of the hell after eons and eons and became a human again.
It seems he wasn't content on one trip to hell! During another lifetime, when he finally got back up to being human, he was again so messed-up that he threw a rock at an Arahant. It hit the guy on the head and made him bleed. His reward? Straight back to hell! Eventually he came up again, and became a great human being in his final lifetime, fully enlightened. So, somehow or other it can happen. I can't tell you how, but somehow or other it did happen for Mogallana.
How long will that take? How long is an eon? Mogallana was down there for a few eons and it took him eons to get back to being a human. What's the length of an eon? Here's an explanation from the scriptures, so take it for what you want, this is what is written. Imagine you have a big rock, a kilometer long, a kilometer high, and a kilometer wide, this great, big, huge square rock. A person comes up to the rock, one time every hundred years and takes a silk hanky, and they stroke the rock, then they walk away. Every hundred years they come back and they stroke the rock. They say that the rock will disappear before an eon is finished. That's a long time!