I understand perfectly. He's trying to antagonize the cop. What's not to understand?
He wants to record the cop losing it, and he'll keep pushing until he does. Completely understandable.
You understand nothing, and seem not to value the freedoms and rights that so many have strived to preserve from enemies foreign and domestic.
1. In the tape, Darrow isn't trying to antagonize the cop.
2. The cop stopped him and asked for his ID. Darrow produced ID. In fact, Darrow complied with every legal request the cop made of him. The cop lost control of his emotions due to Darrow's "bad attitude", not due to any criminal activity on Darrow's part.
2. To my knowledge, there are no "bad attitude" laws on the books in any state. Even if Darrow had spoken to the cop disrespectfully — which he did not — disrespect isn't a crime.
4. Darrow is right to want to record the cop losing control of his emotions, since an armed out-of-control cop is dangerous.
5. The cop in question has a history of criminal activity. He has since lost his job.
6. Since Darrow did not select the cop who stopped him, we can consider that cop a random example of the police officers in that city.
7. Judging by the random example, abusive and corrupt cops are far from rare on that city's force.
8. With this in mind, it becomes likely that other innocent citizens have been abused by the police in that city.
9. It is furthermore likely that most of the cops who have abused citizens in that city will get away with their abuses, since most of their victims have no documentation of their abuse.
10. Therefore, by documenting the abusive behavior (and indirectly ridding the city) of one bad cop, Darrow has likely saved hundreds of innocent citizens from harassment, and has focused media attention upon the city in question, further reducing the chances of police abuse there.
Question: what is it with you and the word "punk"? Do you think that good cops talk that way in real life? I know good cops, and they do not use terms like "punk" when talking about their jobs. Professional law enforcement officers conduct themselves in a reserved, dispassionate, and businesslike manner, not like jailhouse rapists establishing their dominance over the weaker prisoners. If you are trying to sound like Dirty Harry Callahan, you should keep in mind that he is a fictional character. Any real cop acting as he does in the films would end up cashiered, in jail, or dead.