In the case of a homicide, the question is not who is the initial aggressor or who escalated the fight, it is only who escalated it to the level of deadly force.
Zimmerman could have walked up to Trayvon Martin, shoved him a couple of times and bitch-slapped his face until Martin got so pissed he started fighting back. But if Martin started bashing Zimmerman's head against the pavement before Zimmerman drew a gun, it was justifiable self-defense.
If, however, Zimmerman pulled a gun, and Martin reacted by trying to bash Zimmerman's head-in to prevent Zimmerman from shooting, than Zimmerman was the first to escalate to deadly force and it was murder.
Actually in florida's statutes regarding justifiable use of force, there is a statute that DOES concern itself with with the initial aggressor and hamstrings their ability to use deadly force if they cause someone else to use force upon them first. Your assessment is incorrect as it is written in the law.
Not just that, but there are cases, in Florida, that support the concept that even a false movement like reaching for a waistband, can be met with DEADLY force.
The law also allows people to intimidate force in self defense without actually being under attack, being threatened or actually seeing a weapon. What matters is do you believe someone will imminently use unlawful force upon you and the extent of what you think that force MIGHT be.
If you want, I can name a couple cases from Florida to illustrate my point.
Your statement that TM would essentially have to stand there and take it if Zimmerman started slapping him around and Zimmerman can just use deadly force as a last resort if he bumps his head is patently false.