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?

At the very least I expect him after the fact to wonder what kind of person he was becoming and to start working on self-control. But he does not, so instead his fuse only becomes shorter and his lack of control easier to provoke.

Your responses are in accordance with the values Dumbledore and Rowling promote - that seeking revenge is an expression of love, that one can be vengeful and pure of heart at the same time. I lean towards the teachings of Yoda - seeking revenge is the way to the dark side of the force. While wishing ill on someone who harmed oneself or one's loved ones is a natural reaction it isn't a good one. It is better to become aware of such feelings and learn not to be affected by them.

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.

Err, that only makes it worse in my eyes. After the fact, when he had time to rethink his actions he does not regret them because he nearly killed someone who in the past made some threats that at the time he had no way of carrying out?