He didn't. Cad Bane almost fatally shot Cobb in a brief standoff in Freetown, hitting him in the shoulder. He demanded that the villagers allow the Pyke spice trade to operate unimpeded, something Vanth refused. After his new deputy drew his blaster, Bane shot Cobb and then the deputy four times. The marshal collapsed on the desert ground, motionless, as did his blaster-ridden right-hand man. When I first watched it, I thought Cad Bane had killed Vanth. I was gutted, but I thought it was an impressive storyline tactic to highlight the merciless evil of Bane and the Pykes. Of course, at the end of the episode, we learn that Vanth survived and is being healed/repaired by a modifier.