If you read the Daily Mail article new video evidence has been reviewed which likely gainsays the initial claims made about what happened.
Remember too that witnesses are not always reliable and it could be those who said she told her son to keep shooting may have reason to be less than truthful.
If new evidence has come to light which means the DA is unlikely to meet the burden of proof I think the decision is the right one.
If you watch the video of the attack the assailant sounds and acts high or drunk.
Doesn’t matter whether the aggressor was high or drunk. The use of deadly force is no longer justified once the aggressor retreats. In this case, the aggressor not only retreated but was running away when the shooter chased him down and shot him in the back.