The original movie had Han shooting first, but after several iterations of George Lucas changing and rearranging things he made it so that Greedo shoots first.
There's an awful version, I think the first time George made the change, were Hans head moves as if photoshopped by a six grader to dodge the blaster shot. I hear later versions improved that.
So, in fact, this debate is reasonable, if maybe a little pedantic, as it is a minor but not unimportant detail.
George Lucas at least felt it was important that Han shoots in self defense while a lot of people argue that it makes more sense for a smuggler to save his hide than champion morals.
Edit: Although this post is most likely troll bait, I might add, that a lot of people in their 20s and younger now will only have access to the newer versions of the movies, maybe being unaware of the amount of changes that were done to them. Not everyone keeps trac of such things.^^
Atleast on Spike, they STILL show that version. The first time i saw that, my jaw just dropped, what a disgrace.