Trayvon Martin was serving suspension for marijuana possession

[The Mikado]

Again, whatever you say about Martin, the fundamental fact in this case is: Zimmerman started the confrontation, Zimmerman was armed while Martin wasn't, and Zimmerman fatally shot Martin.  Period.  I don't care if Martin was a delinquent (and there's no real evidence he was): even if he was, it doesn't change the previous sentence.

[The Mikado]

...And how is any of this, even if true, a justification for Zimmerman shooting him?

Let's say, hypothetically, that you're right and he's a purse-snatcher, vandal, and drug dealer.  Does that change the facts of the case at all?  He was still gunned down by a man who followed him for multiple blocks and, upon being confronted, escalated a (probable) fistfight to shooting a firearm at an unarmed opponent.  We arrest and convict people for killing prostitutes, gang members, robbers, drug dealers, and other criminals.  As a society, you can't just say it's open season on people that break the law.