Harry didn't quite know how it had happened. He and Draco had been arguing - Draco hadn't liked his involvement in Muggle affairs, had thought that if he was going to be breaking the law, it should at least be *wizarding* law. Smuggling Polyjuice Potion, or something.
The argument had taken a violent turn - Draco had rounded on him, fist raised, and in a moment's fear, Harry had pulled out the gun he kept for use in the Muggle side of things.
Draco hadn't known what it was, had had no idea of the danger he was in. So he'd kept coming, and before Harry knew what he was doing, he'd pulled the trigger.
Harry Potter, the Man who Killed his Lover, took another slug of his whiskey and waited for the Aurors to come.