It's unbeknown what Rotekk's crime was before he was sent to SAR TOP, as it's not revealed in the episode.
Upon arriving on Earth, Rotekk took the form of a human called Johnny Idenasco - an infamous gangster with the Chicago Mafia. Working both for the Mafia's leader Tommy as well as for Zin as a double-agent, Rotekk infiltrated the Mafia's operations (still pretending to be Johnny) and in particular, their involvement in the construction industry - due to Zin's search for the Strada Brac. As Zin began to push into the Mafia's operations, a turf war began to break out between the two groups. Although Rotekk did his best to maintain his fellow gang member's belief that he was on their side, Tommy and his gang had indeed learned of his affliation with Zin and were preparing to kill him later.
Following the killing of a betrayer in the Mafia, Johnny and Tommy's second-in-command Jimmy were getting ready to dispose of the body - only to be confronted by Cole/Daggon. As the pair drew their guns and prepared to shoot him, Cole used his hyper-time ability to disarm both men quickly before knocking Jimmy to one side. Throwing Rotekk to the ground, Cole easily pinned him down before drawing his Collector and absorbing Rotekk's life force.
Despite been temporarily knocked down due to Jimmy recovering before been captured and interrogated by Tommy (learning that the Chicago Mafia already knew Johnny was another traitor), Cole was able to escape his captors before putting Rotekk's life force in his prisoner storage unit. Following the capture of this SAR TOP fugitive, Cole explained to Mel that in order to possibly find out what Zin was after, he would have to infiltrate the Chicago Mafia.
Tracker (TV Series) - Episode 17: A Made Guy