I think it was just AK that was explicitly stated to be unblockable.

'but what is the point of torturing hir?'

Uh, in the first two cases Harry had just seen someone he cared deeply about murdered.

I'm not saying that makes using a Crucio cool, but sheesh. How do you expect someone in that situation to react - to think their every subsequent action through perfectly despite the heat of the moment, or to just throw the worst thing they can think of at the loved one's murderer?

As for Draco's past, I was referring to Harry's lack of great regret later on rather than to his actual use of the spell.