Supposing that was actually what was presumed by Zimmerman, he was proven correct.
According to a witness who was on the phone with Trayvon Martin immediately prior to Zimmerman killing him, Trayvon felt very threatened by a creepy man who was following him around the neighborhood, apparently with good cause. Under Florida law, if Trayvon felt threatened, he had a right to defend himself.
The fact is we don't know exactly how the final confrontation started. But we do know that Zimmerman had no business cruising around that neighborhood playing armed vigilante, and Trayvon had every right to walk home from the store.
You should watch some of the recordings of these incidents.
I have watched many. Including nearly identical scenes wherein a white suspect was not killed despite acting as aggressive as African-American suspects who were subject to violence.
Arguing with a police officer is not a death penalty offense. Neither is being uncooperative. We cannot just give the police carte blanche to declare any hand gesture a possible "reach for a weapon" deserving deadly force.
Are you actually asserting that African-Americans who are stopped by the police are more likely to make "threatening" hand gestures than white men who are stopped by the police?
An officer's job is already hard enough without having someone they're dealing with acting in a way that appears threatening.
An officer swears a duty to protect citizens, and that includes civilians that they are interacting with. Ethically, an officer, who operates from a position of authority and who is authorized to use deadly force, has a responsibility to do everything in his power to not cause harm to citizens, even if in so avoiding giving harm he exposes himself to greater risk. When officers kill a "threatening" African American who has no weapon, that risk assessment was incorrect, and if its happening over and over again, we need to find a way to correct it. And if it is happening in situations with black young men but not white young men, that indicates that not only is the risk assessment being handled incorrectly, there appears to be some bias, conscious or not, influencing it.