So let me see if I can summarize:
1) Guy inadvertently cuts line at an ATM
2) Other (large) guy yells at him for doing it
3) Guy who cut line says "I'm sorry" and moves out of the line
That about it? I left out some of the nuances, of course, but that's how a prosecutor would present it to a jury. Except for the part where 3) is replaced with "The defendant here murdered him with a handgun"
I don't think you're allowed to shoot a guy for yelling at you (or walking up to your car window) in any state in the nation...and that's all the guy did. This story is missing one crucial element that would take it into the realm of acceptable self-defence: the presence of a credible danger to life or safety. "Walking aggressively" may indeed be intimidating, but it's not the same as pulling a knife or punching out your car window.
Given all the information described in this thread so far, you most certainly did the right thing by not pulling a gun on him, since he didn't actually make a credible threat of physical violence. You can't assume all large hulking men are going to rip your limbs off any more than you can assume all scantily-dressed women know how to dance on a pole.
It sounds like you had plenty of time to start shooting if he had actually initiated an assault.