the only explanation that can be given for Greedo shooting first (which im not sure if this is actually said or confirmed by Lucas) is that he did not find it in Han's character to kill someone in cold blood...
he felt it made Han a bad guy and therefore decided he should shoot Greedo in self defense...
i personally find that to be the worst reason due to the fact that Han is that way at the beginning...
which is why its such a surprise to fans when he comes to the rescue at the end of ANH and helps out the rebellion...