A likely sequence of events is as the right rudder load was increasing during the takeoff roll toward full rudder deflection upon becoming airborne, the pilot has diagnosed a left engine failure on the strength of required rudder force alone, he's reduced thrust on the left engine, when that has made the condition worse he didn't know what to do next in the remaning few seconds. The initial slash marks in the roof which is first contact imply a fine pitch blade angle (low torque). It would account for the reducing speed and failure to climb. Because he didn't retract the gear the failure was progressive and not one we train for. As speed increased during the takeoff roll, this would have caused confusion. I'm of the view he perceived he was dealing with a left engine problem which overwhelmed him in the time available.