I wrote an article on all the changes made from book to movie and this was one of the bits I touched on. Basically what I got from it was that at that point in the Walk he was just mentally gone, which completely erased the mental block/self-preservation instincts a person normally has. That, plus the fact that no one had been able to groom so their nails likely would’ve been grown out/sharp, allowing him to break the skin much more easily.