Due to the direction of rotation of the props a yawing tendency occurs when the left engine fails, making it the critical engine
Critical Engine Failed P-factor
Operating right-hand engine will produce a more severe yaw towards the dead engine due to the direction of rotation of the props. More "bite" from downward blade which is further from the center of the aircraft.
Critical Engine Accelerated Slip Stream
Due to accelerated air over the wing it causes greater lift and a rolling tendency.
Critical Engine Spiraling Slipstream
Left Engine Critical, The airflow from the left engine is moving around the aircraft and pushing on the rudder causing a yaw to the left. Right engine has no effect.
Most Unfavorable Weight Factor
Light is unfavorable. An increase in weight causes Vmc to decrease because of the greater horizontal component of lift. With a greater horizontal component of lift (max 5 degrees) less rudder is required thus decreasing Vmc.