Armothy became a part of Morty when Rick Sanchez transferred Armothy's physique into Morty's arm to distract other inhabitants by fighting in the Blood Dome (or the Thunder Dome), whilst attempting to steal a large deposit of Isotope-322. Though Morty was at first horrified, he soon used Armothy to vent his frustration toward his father Jerry for agreeing to divorce Beth. Eventually, Armothy found the leader of the invaders who ordered the attack on his village. Morty, realizing that once Armothy's revenge was carried out he would disappear, tried to get Armothy to delay revenge so Morty could continue to vent his frustrations. However, after deciding they both needed to move on, the two bid their goodbyes before Armothy choked the leader to death.
Although since Armothy wasn't a paramedic in life, he failed to judge whether the leader was dead before 'moving on'. Once the target started to come to, Rick held 's hand and the two finished the job.
"See Morty? Now we're both accountable." -Rick Sanchez