Several issues with what you just posted... Dunn was convicted of 4 charges -- 3 attempted murder charges and 1 charge of firing into a vehicle.
Also, the jury hung on the remaining charges; Mr. Dunn was not acquitted. There is a huge difference between a hung jury and a jury finding a defendant not guilty. In a criminal case, to be convicted all members of a jury must find the defendant guilty, a hung jury means that at least one juror could not find the defendant guilty, and an acquittal means that all jurors found the defendant not guilty. In a case where the jury becomes deadlocked and cannot deliver a unanimous finding, the case is declared a mistrial and the defendant can be tried again with a new jury. And sometimes this actually helps the prosecution by interviewing the jurors after the trial to determine where certain jurors had a hard time of producing a guilty verdict. After those post-trial interviews, the prosecution could use that information to change how they go about their case, perhaps providing more evidence for a certain issue that the first jury had difficulty with.
You say that the Stand Your Ground gives the defendant defense for doing "things like this", which if you mean gives him a defense to shoot because of loud music, then you do not understand what that law is about. The Stand Your Ground provision is a type of self-defense plead that essentially says that a defendant can be not guilty of murder if they perceive that they were being threatened by the victim. Since Mr. Dunn stated on the stand that he thought Mr. Davis had pulled out a shotgun, made a comment something like "It's time this guy is going to get it!", at which point Mr. Dunn pulled out his gun and proceeded to fire. If Mr. Dunn's account is true, then this would be a clear case for the Stand Your Ground to protect Mr. Dunn in allowing him to fire against the threat of a shotgun. However since it was at the trial when Mr. Dunn's fiancee first heard about this shotgun, and considering that the police never found that weapon when examining the victims' vehicle, it is possible that this story was indeed fabricated by Mr. Dunn, in which case it would be one of the best areas for the prosecution to target for their retrial.
I too was a little surprised (and disgusted) about the verdict, but as I heard more about what went on in the actual trial as well as that the jury was just hung on the remaining murder charges, I decided that it wasn't a travesty of our judicial system. It just showed that the prosecution just didn't have enough evidence to convince everyone on the jury, or more likely that Mr. Dunn's account of the night added a twist to the prosecution's case that they were not expecting. But now that they know what Mr. Dunn's story is, they can try to strengthen their case.