Actually, Greedo did shoot. In the scene in question, two shots can clearly be heard, one right after the other. While it is hypothetically possible that Han merely shot Greedo twice, it has been stated several times in canon that they each shot once. Ergo, the question is: did Greedo shoot at Han, miss, and then Han shoot Greedo, or did Han shoot Greedo, who convulsed and pulled the trigger on his own gun?