I like the idea of Han as a cold blooded killer. It adds another dimension to his character that he can care for something such as the Rebellion and someone such as Luke and Leia.
I think we have different definitions of "cold-blooded killer." To me it is someone who kills without a second thought or moral qualm—and probably for very little reason. Defending yourself against someone who is already threatening your life is hardly "cold-blooded"—except to liberals, who always weep for the perp and forget the victim. I have a license to carry, and I sincerely hope I never have to use it, even on some low-life scumbag who is threatening my life over pocket change.
You may have noticed that Han wears the same military outfit seen in the blockade runner sequence at the beginning of
Star Wars. The books may go into detail on this, but movie-goers will be left wondering. Was Han in the military, or does he just like to affect a tough, military air? If he was in some service, was it voluntary or conscripted, and was he honorably discharged? If he was not honorably discharged, what were the circumstances?
My own interpretation?
Han had been in the military for his home system, which was annexed by the growing empire. At that point he got out when his tour was up, or maybe assaulted some imperial CO he took a disliking to. Scrounging for some kind of existence, he won the
Falcon in a card game (as revealed in
Empire). I would assume he made one of the biggest bluffs in his life and got lucky by winning.
After that he set up shop taking any kind of work that might p*ss-off the empire, especially since even "honest" work was growing increasingly difficult under their oppressive rule. Unfortunately, this meant working for the most vile criminals to get the highest payoff. Caught between a rock and a hard place, I can imagine the cynical Solo having a very weak moral rudder, reducing his horizons to just himself and Chewbacca.
Then he ran into Luke, a youthful idiot with naive and idealized notions of "rightness," stoked by the mystic old fool. Odds are Han is not old enough to have ever met a Jedi in their heyday, and thus bases his opinions on aging folklore. After all, how good could they be? They were wiped out in short order. Little did Han know that Kenobi had been at the center of the action, once upon a time. (Incidentally, Kenobi's age in the prequels does not dovetail very well with Luke's apparent age in the original movie. Kenobi should have been at least 20 years older in
Phantom Menace.)
So Han is not a bad person, but he is not a good one, either. I think he came back more for Leia than for Luke, but either way, the rebellion gave him a sense of purpose, of belonging. Maybe he was impressed by Luke's brass in rescuing the Princess, and then taking on the Death Star. That's crazy in any book, but it might have been enough to convince Han that it's better to go down fighting than cowering and running.
All of that is lost if Han graciously allows Greedo to fire first, expecting that he will miss at point blank range. (And he'd have to miss because only in the idealized fairyland of Hollywood are the "bad guys" such lousy shots.)