Your claim that Zimmerman drew his weapon and tried to make a citizen's arrest doesn't follow from the evidence reported.
If Zimmerman drew his weapon BEFORE he was beaten, why wasn't Martin shot as he attacked Zimmerman?
One (or more) Eyewitnesses report that Zimmerman was beaten BEFORE Martin was shot. Evidence supports the Zimmerman's contention that he was decked by Martin and his head was being slammed into the ground. (Broken nose, injury to back of head, grass stains on back of jacket.)
If he called police and lost sight of Martin during this call, then upon exiting his vehicle he could have easily been surprised by Martin. (Vehicles provide limited visibility and one is most vulnerable entering/existing a vehicle.) A verbal confrontation ensued, culminating in Zimmerman being clocked. His injuries are consistent with these events.
I can understand one's reluctance to volunteer for neighbor watch duties. That's fine. It can be dangerous. But Zimmerman did volunteer and was observing a suspicious character. No harm. No foul. We can criticize his lack of stealth from the comfort of our armchairs. However, when a blow was struck, that changed the dynamics of the encounter. The slamming of his head on the ground could easily result in grievous bodily injury, justifying the use of lethal force.
If Martin spied Zimmerman's weapon carried in a belt holster and attempted to wrest the weapon from Zimmerman, then Zimmerman had no choice but to maintain control of his weapon. (I'm guessing this is when the weapon discharged.)
I do not know what kind of training you've had, but here in AZ the CCW courses are limited. Sure, they run through several shoot/no shoot scenarios, but that is hardly adequate. Post CCW training is highly recommended.
Most shooting classes are just that, shooting, not necessarily combat. However, there are many that can prepare a citizen for some likely scenarios--Street Survival, Tactics and Techniques, Close Quarter Combat, Edged Weapons, Baton/Martial Cane Concepts, Combatives--all help to instill needful survival skills. However, the one area that I think is lacking is material I've seen addressed in Kane's Surviving Armed Assaults: A Martial Artists Guide to Weapons, Street Violence, and Countervailing Force. Zimmerman may have benefitted from a better understanding of the force continuum. Kane presents a measured response at each stage where one employs a different skill set: (1) presence/authority, (2) voice, (3) empty-hand restraint, (4) non-lethal force and (5) lethal force.
“Your claim that Zimmerman drew his weapon and tried to make a citizen’s arrest doesn’t follow from the evidence reported.”
I did not claim that. I said he “could have”, for all we know. What we do know is that Zimmerman displayed extremely poor judgement prior to his physical confrontation with Martin by getting out of the car and pursuing Martin on foot, at night, in the rain, while armed. There’s no reason to assume that Zimmerman’s judgement became any better during his foot pursuit and the time leading up to the physical altercation.