This is a false equivalency, it (poorly) attempts to present a scenario in which everything is the same except the background of person that gets killed, but the entirety of this description is in fact a completely different scenario.
For starters, zimmerman did not "chase" martin. He kept a very firm distance and even told the dispatcher that "he's running, he's gone, I don't see him,"
Secondly, addressing his specific self defense claim, being kneed in the balls, peppered sprayed, or tripped are all one-off instances. They are not comparable to being continually beaten for a minute straight, with 3rd party witnesses begging for trayvon to stop only to be ignored.
You certainly aren't going to be fearing for your life or grave bodily injury if you are kneed in the balls.
Nor are you going to be fearing for your life or grave bodily injury from being pepper sprayed.
Nor will you be "fearing for your life" if you get momentarily tripped. Once you are tripped it is over.
If a women that was being shady, walking in between houses, and then ended up jumping someone that had done nothing unlawful and started beating the shit out of them, (and was physically capable of doing so to the extent that her male counterpart would be) then absolutely she should get a lead salad. Bullet train to the graveyard.
But this is not the type of description being given.
As such, the situation presented is not comparable. The situation zimmerman was in was one in which he had a reasonable fear of death or grave bodily injury. A jury agreed upon by both parties, one in which was even looked upon favorably by howey himself until they told him something he didn't want to hear, all agreed with zimmermans account. They found it reasonable that he may have fear for his life of grave bodily injury, and that he needed to stop the violent physical confrontation perpetrated by trayvon the attacker.
So to answer this fanciful strawman, no one would have a right to shoot someone for being tripped.
Too bad trayvon didn't simply trip zimmerman though.