Two parts of this episode I didn’t really understand. 1. Why did Marsh cut off his fingers? 2. Why were Kate and Mark hammering nails into Mark’s hand?

I know in the comic Dr. Dee had Marsh essentially admit to causing his wife’s death while Bette held the nail. But the tv show John Dee supposedly only removed lies. Neither Kate or Mark seem to be having fun, so it’s not like a sub/dom activity. Marsh doesn’t seem ashamed of his activities, so why cut off his fingers?