A civilian military trainer pilot reported during their takeoff roll from a non towered airport they observed an aircraft back taxiing opposite direction on the same runway. They rotated immediately and offset to the side of the runway.

Narrative

During a very busy day at ZZZ; there were multiple aircraft in the pattern; war birds doing low passes over the runway; nonstandard comm on CTAF; taxiway 1 between 1X and 1Y was closed; and the runway is on a hill where aircraft holding short at Runway XX do not have clear visibility of aircraft at the far end of the runway. The runway in use at the time of the event was Runway XX. I was number 1 at the hold short on taxiway 2 for a Runway XX takeoff while a Pilatus PC-12 was on final. The PC-12 landed and it appeared he had exited the runway. I subsequently made my CTAF comm 'NXXXXX; Departing Runway XX; VFR to the East' and proceeded to taxi onto Runway XX and begin the takeoff roll. Halfway through my takeoff roll I noticed the PC-12 back-taxiing on the runway. He directed 'Aircraft on takeoff abort' however I was now at a high speed and near rotation and thus continued the takeoff and sidestepped for a very comfortable and safe pass abeam the runway. I elected not to abort as that would have been more dangerous with two opposing aircraft on the runway with one at high speed. Subsequently the PC-12 pilot radioed apologizing for not making a back-taxi call on CTAF. The lessons learned are I should not have assumed the PC-12 had cleared the runway; recognizing the PC-12 owns the runway after landing and for the entirety of the back taxi; the PC-12 should have been monitoring CTAF and heard my takeoff call and subsequently radioed that they are still on the runway and the PC-12 should have made a 'Back-Taxi' call on CTAF regardless.