Nicely written; it had a different tone from most HP stories which I quite enjoyed.
I wish we'd gotten to learn more details of the story's background. For instance, did many of the victims share one or another detail in common with Snape as a youth (dark hair, very thin, gangly, etc)? Perhaps Rowle was trying to get revenge against Snape symbolically. ;) What did Rowle do once he followed Snape's example (again) and changed sides? Why was it only now that he fell into his old habits and started killing again?