The amount of despair Neville shows in such a short section worked its way into my psyche as I read, and i'm afraid I may not make a lot of sense here. I couldn't imagine from the beginning how the tags could apply, and the grittiness of the evolution was very well done.
The whipping scene was, for lack of a better word, ugly (its a complement). The raw quality of both their emotions, and the implications of Severus' prior violent sexual experiences was uncomfortable in the best possible way. When Neville considers picking the whip back up, I was hit with chills at the implication of real damage. I can only hope the two have the opportunity for a less volitile friendship in the future, as I believe they will still need each other.