Let's ignore the original presentation of the scene in which Han makes a preemptive kill and follow the altered scenario that Greedo shoots first. He's a bounty hunter holding a pistol aimed at Han with the muzzle only about three feet away. He fires and misses Han by about a foot, allowing Han to leisurely fire back. It's ridiculous. Not to mention the terribly unconvincing editing job done in the revised version of the scene.