he only reason not to free Bellatrix Black should be the danger of her attacking other people later on, and that's the point on which he should seek reassurance from Quirrel (re: what they are going to do with her once freed).

Even if Harry's not a utilitarian, I'd still like him to be smart enough to realise that this is still an important practical question to ask.

But he's only 11, so I only hope that EY will let him realise his mistake later.