Perhaps, though I recall Imperius being used a few times in the book; I was a bit tired, so I forget all the details.
Otherwise, another possible 'out' for Harry would be that there was no legitimate government once the Ministry had fallen. That would probably protect him legally, though it should have no impact on the moral wrongness of such actions. And use of Unforgiveable Curses on innocent sentient targets would seem to be morally wrong whether or not such targets were human.
This is, in fact, one of the central themes in the books. Many wizards, like Umbridge, see nonhumans as subhuman. They don't really believe them to be fully sentient, or belonging on the same level as humans. At best, they are things which need human protection to keep them from hurting themselves.
Contrast this with someone like Hermione who fought for (what she believed was) the well-being of house-elves. Contrast this with other Wizards who treated non-human sentients as equals. In many cases, the way a Wizard treated non-humans was used to demonstrate their character.
As far as Harry goes, I see him using the methods he did at Gringott's only as a last resort. Destroying that horcrux and vanquishing Voldemort topped off everything else. He couldn't allow anything to stop him. I'm sure using the Imperius Curse and releasing the dragon were the last things he wanted to do. He knew innocent people might be hurt. Then again, if he failed, all those people would end up dead anyway.
I don't see the goblins at Gringotts as innocent. They knew perfectly well that Bellatrix was a Death Eater and acting in the service of Voldemort. They are about as innocent as the Swiss bankers that did business with the Third Reich.