The MU-2 dowan’t have ailerons, it uses spoilers.
In an engine out condition, you bank toward the good engine, but end up losing total lift due to the spoiler killing lift on the wing with the operating engine. Very high power setting is needed to offset this, and a turn away from the bad engine may not be possible due to not having enough power available to sustain level flight.
This will cause either a drop in airspeed or altitude. Neither are good if at low altitude maneuvering to land.
WAG: He was too low to dead stick it, and the available engine only caused him to depart controlled flight not fly the aircraft.
Note the odd configuration of the wing control surfaces. Most aircraft have an aileron on the outer portion of the wing, and a flap inboard. This has full-length flaperons, by the look of it.
A high performance and unforgiving design.