I saw it more as a statement of how much Chuck changed and how far he is willing to go to "get" Jimmy.

Now that he has the tape, neither of Chuck's choices are good.

1. He can go to the police or state ethics board, and while he might severely embarrass Jimmy, it's pretty easy to explain his way out of and exposes Chuck to all sorts of scrutiny.

2. He can use the tape to blackmail Jimmy, to try to get him to stop practicing law or to at least stop using the McGill name so as to not further "sully" it. But if/when Chuck makes that choice, he gets "down in the dirt" with Jimmy, the exact place where he refused to go earlier this season when Jimmy offered to stop practicing law if they stopped punishing Kim.

I looked at the end as the show's way of saying that Chuck is now prepared to become a criminal, and sink just as low (truthfully lower) than Jimmy all in order to vindicate himself and vanquish his brother.