Well there is no question that he killed her, the real question is did he intend to kill/shoot her?
You need to remember that the area that he lived in was not exactly a safe area - it was a gated community, but there is a lot of crime in the city and someone could potentially make it past the gates/guards.
He was also found not guilty for murdering her, and was found guilty of what is essentially manslaughter (accidental killing).
I honestly have not seen enough evidence to make a good decision if he intended to murder her. If you feel that he did purposefully kill her then yes, the sentence is a joke.