I'm not even sure about that.
He identified the individual as suspicious.
He called the police.
The 911 operator told him not to pursue.
He pursued.
An altercation occurred which led to him discharging his firearm.
Would a reasonable person have followed this same sequence? Did Zimmerman's decision to pursue in light of the 911 operator's comments move him into the realm of "instigating conflict" thereby negating any possible self defense claim?
I think Zimmerman messed up the moment he ignored what the police told him to do. I think that's where he lost the right to claim self defense.