This Florida "stand your ground" trial has become a much larger issue than it might have been if the media had portrayed it differently. Instead of being a matter of self-defense it became a matter of race. Even then, when most of the nation was outraged, it did not need to be escalated more by President Obama.
His speech, while neutral, was still a hit to George Zimmerman. After Obama gave his speech sympathizing with Trayvon Martin's parents, he, perhaps unknowingly, sided with the Martins. With most people believing that the President was on Martin's side, they rallied in support of Martin.
Since he helped facilitate this push against Zimmerman, he should apologize for coming across as partial. Everyone makes mistakes, but when they happen apologies are necessary.