That's all the opening Wash needs. He's not hiding far from his trap, and Cashmere's angle of approach and relative carelessness has given Wash a position behind him and the advantage of surprise.

There is absolutely something to be said for sneak attack damage. Wash surges forward and reaches around Cashmere, plunging his knife into Cashmere's neck and dragging the blade across his throat. "Say hi to Felix for me," he snarls, knocking Cashmere's blaster off its holster and tossing it to the side. It clatters to the ground next to the plastisteel sword that slipped from Cashmere's grip when Wash stabbed him. Now all he has to do is take care of the other one and he'd be home free.