At some point, most likely due some reason to his grandfather Rick, Morty became obsessed with stopping fascism at all cost. He got himself a "Hitler-measuring gun" from his grandfather, which he took along with a portal gun and a Laser gun. From then on, he went from dimension to dimension, fighting and killing everyone who measured high on the "Hitler-measuring gun".
In one dimension he met his C-132 counterpart doing the exact same thing. They both scanned each other and prepared to shoot, but Rick shot this Morty first.