My explicit hope is that Harry is doing that intentionally, after carefully determining whether the troll needs to die or not. For (what I think is) the purpose of increasing his chance of success. He seems exactly the kind of guy who'd temporarily manipulate and self-decieve himself for high instrumental utility, and has demonstrated the ability in the past (with the Dementors in TSPE). The main competing hypothesis is rationalizations from an influential Dark Side, which seems less likely. (~10%)