Greedo actually threatened either to kill Han himself or to take him to be killed in the conversation we saw, so it's not even equivalent. Han is on firmer moral ground just shooting Greedo, the way he did in the 1977 version.To me the equivalent is: The Rebels might have thought that the Death Star was coming to blow up Yavin IV, but they didn't know. So why launch a preemptive strike on the station, murdering (looks it up) over a million people on board?
Plus, there's the cardinal rule oft repeated in gun-safety courses, that you don't point a gun at someone who you don't intend to shoot. Either Greedo was breaking that rule or....