It's difficult for many positions in football. Let's say you have a defensive linesman. What is a high score? Does he get a high score only if gets through the offensive line? What if he makes a tackle on the RB? Which of these gets a higher score? It's subjective and/or based around the situation and analyzing every play from the perspective of every player would be time-consuming.
One way to do that is really simple (But purely objective) is to grant every player on the defense a score based on how many yards the offense got (and just score the offense for yards).
Offense loses 5 yards. Every player on offense gets -5, every player on defense gets 15.
Incomplete Pass. Offense gets +0, defense gets 10.
Offense gains 10. Offense gets 10, defense gets 0
Offense gains 40. Offense gets 40, defense gets -30.
It's a stupid system that won't tell you much about individual contribution, but it's objective. You might want to move the numbers around (i.e. offense only gets +5 when they gain 10 yards or whatever).