The recent conversations Harry has with Dumbledore seem to imply that Harry is becoming what tvtropes calls a Knight Templar: a zealot who is willing to burn villages in order to save them. Such a person will very easily commit great evils in the name of "The Greater Good" (whatever that may be). I agree with Dumbledore on this. Harry has no brakes; he is willing to escalate his counterattacks without limit, collateral damage be damned. This may not technically count as "Dark" according to the dichotomy set up in the narrative, but it's still a completely evil way to act.