Rodney King's central forehead is also contracted. This portion of the forehead contracts when we feel emotional or physical pain, either directly in the first person, or indirectly for others via empathy. Be very wary of what the mouth does during such times however. When the mouth nonverbals are congruent with these specific emotions - (with no signs of smiling, even slightly) - usually with some component of a frown or fear - such body language is highly consistent with the emotion of sadness, or physical pain.
When King says, "...you know people have forgiven me for some things that I have done, you know, um, and so, I wouldn't be able to live with myself if I wasn't able to forgive the officers ..." - he was being very sincere.