Never mind the “Who shot first?” stuff; THIS is the defining moment of the Han Solo character:
Everyone else was just like: “Oh, fuck: It’s Vader!” and standing around in shock and terror when Vader was revealed. Even Chewie.
Most people, I suspect, would have that reaction.
Meanwhile, Han’s first reaction, instinctively, in less than a second, was to grab a gun and try to flat-out end the guy.
He failed, of course. But God Damn if you can’t appreciate the effort.