I know violence from the nursing side of things.
After a head injury you aren’t thinking too clearly.
His head was bloody in the back and bloody in the front.
Likely he was dazed and in a bit of emotional shock as well.
You are of course correct. Plus - killing someone at short range - emotional on top of physical trauma.
It is the “blood on the back of the head” that seals the confusion. AFTER a shot like that - the tendency is to go on autopilot - fight or flight. I could fight like that - but I could’t do algebra. Probably - fighting is about all I could do like that. It triggers a very specific feeling. A much reduced instruction set running in the brain. It’s been a while since I’ve had one fo those - but I bet there is an adrenaline/endorphine event that occurs. COMPLETELY different from a frontal lobe impact.
The cops would know this - same as you.
There is one aspect you leave off - the confusion of both guys. (It really is just a “two dumb guy” event.) - Martin probably assumed he was being followed by some Hispanic hard case. Zimmerman THOUGHT he was dealing with a career criminal.