The case for manslaughter is simple. The jury disbelieves Zimmerman’s claim that he was in reasonable fear of death or serious injury. Zimmerman’s injuries support the idea he feared serious injury, but they don’t require it. A finder of fact could certainly say, “Eh - a brawl in the street, some cuts and scrapes, but not lethal injuries, and I’m sure Zimmerman knew that was all that was going to happen.”
There goes self-defense.
Now we have Zimmerman shooting Martin, and not in self-defense, but with no intent to cause death and no ill-will, malice, or hatred. That’s manslaughter.
A civil trial is not a violation of double jeopardy. Double jeopardy applies only to criminal cases. The main differences between the two are the parties and the burden of proof. In a criminal trial, the parties are the accused (the defendant) and the government, be it state or federal (variously styled “The State of xxxx,” “The Commonwealth of xxxx,” “The People of xxxxx,” or “The United States.”) The government must prove that the defendant violated an existing criminal law, and must prove this by showing evidence that each element of the law is proven beyond a reasonable doubt.
In a civil trial, the parties are the person who claims he was wronged (the plaintiff) and the person who is alleged to have done the wrong (the defendant). The plaintiff must show that the accused committed a tort, a civil injury, by showing evidence that each element of the tort is proven by preponderance of the evidence (basically, showing it’s more likely than not).
The penalty for conviction of a crime can be a fine, paid to the state, imprisonment, or death.
The penalty for being found liable in tort is money paid to the plaintiff, to make him whole for the injury the defendant caused, and possibly money paid as a punitive measure as well.