Do we agree than contributing actions could lead to criminal liability? If Zimmerman started the fight, and came out on the short end, legitimately defending himself isn't murder, but there might be something there. Not suggesting that happened.
If I sell a firearm to someone without documentation, required or not, I could be subject to civil or criminal penalties.
If I sell a firearm to known criminals, who use it to kill anyone from a Border Patrol agent to a Police Chief to hundreds of citizens, many probably criminals, of another nation, I'm guilty of nothing.
Go figure.
On the matter of civil liability, the standard is preponderance of evidence (or, who has the most). As we know from the OJ trial, you can meet this standard and not meet the standard for criminal conviction of beyond reasonable doubt. But, what money could they get from George Zimmerman? Even if he were found liable, he’d simply declare bankruptcy and walk away. He wouldn’t even need a lawyer. Just defend yourself pro se and let the other side spend hundreds of thousands of dollars for the possibility of nothing.