When a crime is committed, the people's interest in an ordered society is represented by the prosecution, which pursues its task (without passion or prejudice) within an accepted and respected framework of law.
Our reverence for the law and the assumption of its equal application (at least in theory) are part of the social contract that holds us together as a society. When that contract is violated, it's an affront to us all. That is, I think, what lies at the root of the anger at Donte Stallworth's punishment, or lack thereof.
We call the punishment of a crime the perpetrator's "debt to society" for a reason. It is not his or her "debt to the victim," because in theory, it is society and its code that have been wronged. This is what keeps our system from descending into "eye for an eye" justice. The legal system is there to protect us from ourselves, from each other, and from our natural revenge instinct. Without it, we'd all be killing each other off in vendettas.
The redress of personal grievances is settled lawfully in civil court. The fact that Donte Stallworth made a financial settlement with the family of his victim should have no bearing on his criminal sentence. We know this, if not because we are familiar with the law, then because we feel it in our guts as members of a collective group with a stake in preserving our code.