If I could jump in here, I personally don't think that Greedo was absolutely definitely 100% going to shoot Han. I mean, if he had shot and killed Han right then and there, he'd have a 170 lbs. dead body to drag all the way from the cantina to Jabba's palace. That's, what, at least ten miles as the crow flies? Greedo doesn't look strong enough. No upper body strength. He knew that wasn't going to happen. Unfortunately, he wasn't sharp enough to march Han right out of that cantina the moment he confronted him. Instead, he let him sit down, relaxed the "keep your hands where I can see them" rule, and, well, that was that.

And even if Greedo wasn't going to take him to Jabba, even if his intent was to collect Han's cash and "forget" he found him, Han clearly stated that he didn't have the money on him. I suppose Han could have been lying, but the only way Greedo would have known for sure is if he had kept Han on his feet and searched him. If he had shot Han dead with empty pockets, he would have been totally fucked. But, as Darth Chaltab put it, our green friend was too busy gloating like a Bond villain to think any of these scenarios through.

That's what I think, at least. I could be (totally) wrong.