By far the most common explanation I've seen is that he was making a Marxist statement with his raised fist. That would scarcely seem to make sense if he was playing the innocent patsy card.
Since he squawked about police brutality, my guess would be that he was just showing how harshly he was being treated. He recognized a photo opportunity when he saw one.
Why not just hold his hands straight out, victim-like?
I particularly like the clenched-fist aspect, don't you?
Is it "harsh treatment" when a violent murder suspect is subdued (without being beaten up), handcuffed, and taken to a police station?
Just curious: Do you think "Russiagate" was a hoax?