According to a Lawyer, Han Solo Had a Legal Right to Shoot Greedo First 

Ethan Anderton, for /Film:

A lawyer recently tossed up a post at a law blog […] that says Han was fully justified in shooting Greedo first, essentially proving that there’s no reason to frown upon the smuggler for shooting the bounty hunter first.

Clarifying that Greedo intended to kill Han Solo at the cantina anyway and that Han didn’t really have ample opportunity to retreat from the dangerous situation, here’s what the post over at Legal Geeks (via Vulture) says:

“Without a doubt, having a blaster pointed directly at Han put his life in danger. Additionally, Greedo’s statement, “That’s the idea. I’ve been looking forward to this for a long time,” communicated Greedo’s intent to kill Han. Shooting first was seemingly the only way to prevent Greedo from using deadly force himself. Regarding the retreat issue, Han was already at gunpoint and cornered in the booth when Han shot Greedo. It is unlikely Han could have retreated with his back to the wall and in a seated position. Shooting his way out appeared to be his only option. Finally, reasonable belief wouldn’t be hard to prove. Han was in Mos Eisley Spaceport, a wretched hive of scum and villainy. Greedo had his weapon pointed at Han the entire time, with Han cornered in a booth. This should be sufficient to show the reasonableness of the threat to Han’s life.”

Anderton again:

[You] can rest easy that Han Solo isn’t just a senseless killer and George Lucas never should have changed that scene to begin with [emphasis added].

There are a lot of things Lucas just shouldn’t have done.