Again, not attempting to excuse or justify this hiliariously weird change by Lucas, but to attempt answering your question, I believe this is what he was intending:

Greedo at no point appears to believe Han is a threat, which is why he’s talking so big.
Han is content to let Greedo believe he’s unarmed, which is why he’s surreptitiously getting his gun ready.
Greedo keeps talking bigger and bigger
Han, with his weapon free, goads him to see what he’ll do.
Greedo now says “MacKlunkey!” which is apparently actually “Ma Klounkee” - Huttese for “This is the end of you.” or something like that.
Han shoots first.

So I think the intent (again, not saying he was successful at all) was to make it even clearer that Greedo was out to burn Han, and to have the scene play more like an actual showdown instead of a sneak attack. In this way, Greedo’s boldness and stupidity in needing to get the last word is what gets him shot.

Fourth time’s the charm, I guess? (It isn’t)