Harry has never had a problem killing suprnatural beings OR humans, if given the correct reasons.
In retrospect, he was silently horrified about how many enemies, human or otherwise, never seemed to surive encounters with him. This was after marcone pointed it out to him.
But Harry would have no problem pulling a gun on a human and shooting them dead in the middle of a battle, or to protect someone.
The only time he ever really REALLY felt bad were after executing Cassus, Luccio(Corpsetaker), and Susan. He either did it cold or without thinking, and that is what scared him.
He had perfectly legitimate reasons to kill the two fae at his birthday party. After setting the standard, backing down in front of preditors is a sure way to become prey. Numer 1 died because they questioned him. Number 2 died because they questioned him. He was protecting humans like Sarissa, and he couldnt afford to come off soft, or he may find himself turned upon by winter.
He wouldnt feel bad about it if he killed 100 more, for the right reasons.