Again, there is no defense of this killing, even if the teens in some way verbally provoked the altercation that then escalated.   But, your assumption that the senseless factor makes for outrage doesn't make good sense to me. 
Whereas it IS senseless to kill for a petty argument, if we knew the motivations of the referenced black-on-black teen murders, they would likewise be senseless to most people too. 
Racial murder is senseless.  Drug deal murders are senseless.  Ego and testosterone motivated murders are senseless.   Fights over girlfriends ending in murder are senseless.  Murders committed during theft are senseless.  
My point remains that the great number of other senseless killings are appalling and a stain on our culture even more than the much more rare racist killing, if indeed this was about race or culture clash.  And yeah, it is sad that someone would attempt a stand-your-ground defense, but lawyers often do outrageous strategies.  Even if the killer was somehow able to get off with this unreasonable defense, it wouldn't make it typical, and it wouldn't outshadow all the tragedy of black-on-black murders.
Somehow, it is fashionable on JUB to pick a state like Florida, make generalizations about the entire populace based on a crime like this, and then portray it as some kind of hotbed of racial conflict.  There must be a more compelling argument than one case.   And, it seems ironic that taking offense at racism in a case like this then becomes the basis to stereotype Floridians.  Think about it.