Except you don't know that. Again, to quote the hypo:I would go after the attacker. Then he won't kill more people. If I save the wounded man, I'm leaving a source of wounded men alone.
For all you know, the victim deserved what he got: the attacker is an avenging angel punishing the "victim's" murder of a helpless child, after sexual exploiting several nuns . . . etc.Assume that the options are generally equal - the man running away isn't bin Laden or someone else it's specially important to catch, the wounded man isn't a known philanthropist who will give you a million dollars for saving him
Another way to look at the problem: what would the victim want you to do? (Or what would you want done, if you were the victim?) Maybe the victim really, really wants the attacker brought to justice, but odds are, he'd like to live, more.