If the cb was pulled for the data voice recorders i am confident the perpetrator would have made a blunder in his grand plan as the initial cockpit struggle would be preserved on the cvr
Your theory that a cockpit struggle would have been evident assumes both pilots were in the cockpit at the same time. In the case of the Silk Air MIA 185 B737 suspected suicide crash, the circuit breakers for the CVR and FDR were found to have been actuated by physical means not an electrical fault. it was thought the most plausible scenario was the captain conned the co-pilot to leaving the cockpit on the pretext of sending him to the cabin to investigate an event (eg leaking door seal). It was speculated that once the co-pilot had left the cockpit the captain locked the door, and pulled both FDR circuit breakers (that action activates the Master Caution light).
It was considered very doubtful the captain pulled the dual circuit breakers for the FDR while the co-pilot was still in the cockpit, as the illumination of the Master Caution Light would have alerted the co-pilot something dodgy was being planned. There was evidence the captain had already previously pulled the CVR circuit breaker situated behind his own seat and out of the forward sight of the co-pilot. Easy done as he had practiced that before on the ground on a previous flight.
Pulling the CVR circuit breaker (B737) does not activate the Master Caution and the co-pilot would have been unaware the CVR circuit breaker had been pulled by the captain who had himself left the cockpit a few minutes earlier on a visit down the back.