Upvote:3
I see two potential approaches you could take.
The closest thing to a single event that comes to mind is the apparent assassination of Fred Hampton. Regardless of who fired the first shot, you could easily argue that this was effective repression that weakened the movement. You could also argue that it occurred because of the Black Panther Party's posture on self-defense (although some might say it had more to do with other aspects of their agenda.)
The other approach might be to look at what lead the civil rights movement toward nonviolence in the first place. Here is an article which "explores how blacks in the rural South tried to defend themselves before civil rights" and then how that changed over time. You could say that current of self-defense actually persisted well in to the 1960s in in the form of The Deacons for Defense and Justice. Looking at it this way, it might actually go against your thesis, suggesting that nonviolence and self-defense actually went hand-in-hand.