Of course that Orochimaru-lookalike decapitated the lizardman. My guess is this: Sophisas struck him in a non-vital spot, and such an injury is just incapacitating for lizardman biology. But that means he could've ratted about the details of what happened to end up with him captive, including whatever deal he made with the regent (which would probably be grounds for treason). So... he was killed under the pretense that he could've been playing dead, to ensure that the ploy remains secret.