It's not known what Reta's crime was that caused him to be imprisoned on SAR TOP. However, it's likely that the reason was political and not violent - as Cole/Daggon stated during his tracking of Lontoria.
Upon arrival on Earth and taking over a human body, Reta began to work for Zin - a dangerous Vardian who was using the prisoners to accomplish his own ends in his pursuit of a weapon called the Strada Brac (a deadly device also known as 'The Universal Annihilator'). Due to Cole continuously causing issues with his plans and now standing directly in the way as they neared completion, Zin had decided to take matters into his own hands. Using the prisoners, Zin had a hand-held weapon designed and built that could disable a Cirronian's powers with a single shot - essentially making them human, like Lontoria had become from using her powers to save Cole some time back. The way the gun did this was by neutralising a Cirronian's negative energy particles - thus neutralising the positive as well and making them more or less as weak as a human with their powers gone.
Calling Reta into his office, Zin questioned him on how many Cirronians had come to Earth - to which Reta stated that aside from Cole, it was just him and Lontoria. Zin then declared that he felt it was "three Cirronians too many" before picking up the gun and shooting Reta with it. Sure enough, Reta's Cirronian abilities were completely neutralized - proving to Zin that the weapon worked and that therefore, it would be his ace in stopping Cole and achieving his desire for power.
When Reta (weakened from the shot) questioned Zin on what use he was now that his powers were gone, Zin replied that he was no use at all and could do only one more thing for him. Putting down the weapon, Zin then picked up a handgun. Simply saying the word "Die", Zin then shot Reta in the chest at point-blank range, which killed him. Zin and his followers then proceeded to continue their mission.
When Cole and Nestov prepared to infiltrate Zin's base to retrieve the weapon that Zin used to neutralize Reta's Cirronian abilities, Cole stated (when Nestov asked about Cirronian criminals due to a pair of guards outside the base) that there were only two Cirronian fugitives - the first been Lontoria, who had already lost her powers earlier, and Reta - who Cole states would be too valuable to use as a guard.
Tracker (TV Series) - Episode 21: What Lies Beneath