How does his being armed as he walked back to his truck have any bearing on the fact that Martin
Why did he get out of his truck in the first place? To follow Martin.
How does his being armed as he walked back to his truck have any bearing on the fact that Martin - who had already made it to his destination - doubled back, and ran up behind Zimmerman to sucker punch him and begin the physical assault from which Zimmerman had to defend himself?
Yeah, that's one version of the story.
So your saying Zimmerman's been following this guy who he views as a dangerous threat, then he even testifies the guy notices he's being followed, and then a few minutes later he's walking along with his guard down and gets jumped from behind?
Carrying a pistol doesn't magically control people who are sneaking up behind you to attack you.
He wouldn't be doubling back and sneaking up behind you if you weren't sneaking around following him around in the first place.
Does following someone around somehow justify being attacked? Of course not, but it robs you of credibility when you say "I was just minding my own business, walking back to my truck, when he jumped me." I have a reasonable doubt about the truth of that claim. Maybe its true. Maybe its not.
We need a trial to sort it out. I'm not saying Zimmerman is guilty and I'll assume the jury makes the right decision based on the evidence presented at trial, regardless of whether they acquit or convict.