Definitely very well written. An interesting stand point that we got a glimpse of in OotP. Harry definitely has a lot of anger and you do a good job of conveying that. You include events from the book well and the piece flows nicely as a whole. The only suggestion I would have would be to leave out the descriptions of what he's doing in the brackets. It's a little distracting and doesn't really fit with the format of the story. Try describing it normally instead.
Besides that awesome job and a very good first piece of fiction! Much better than my first was, trust me ^_^ Keep writing!