Background
Born on a remote Imperial colony, Torias was constripted into the Imperial Army at an early age.
It was in his training that he proved to have a peculiar aptitude for long range rifling - becoming highly proficient quickly. Over the course of his service he gained a decent amount of renown as a crackshot, until he was offered a position in a top secret task force.
In order to properly join the team, he had to have no record, and as such was supposedly killed in a rebel raid.
This was an off-the-books Imperial kill team, and as such Torias was frequently tasked with taking out high value targets all over the galalxy.
During his time he became quite close with the members of his team - Erazmus was the field medic, always level headed and calm under pressure, Lorgar was the stealth assasin - seemed to always beon edge and never fully letting his guard down.
Finally there was Kharon, the bruiser, no one in the party quite matched his raw physical power, but he was fiercly loyal as a friend and comrade.
After a few years of getting tasked with high value assasinations, either war leaders, political rivals, or turncoats - usually very simple jobs - all supposedly enemies and terrorists of the empire and thus of peace itself.
One day we get given a peculiar assignment; allof us, barring Lorgar, were called into Erazmus' chamber, where he revealed to us that Lorgar had been secretly been sending credits and information to rebel factions; as such he was our next target.
Torias had his doubts, he did not believe that his firend and compaion of many missions was a traitor. However, no one else seemed to have similar reservations as they quickly moved to eject Lorgar out the airlock before Torias could object.
In an attempt to quell his concerns, Torias scoured through Lorgar's belongings, trying to find as to why he had betrayed them. What he found was that Lorgar had been sending credits to his own, war-ravaged family back on Batuu, and that the "information" being leaked was simply audio recordings he'd sent them to let them know how he was doing.
This struck a chord in Torias, who had not known family, to see someone punished so hard for trying to help theirs, then being so swiftly turned on by their supposed friends.
As he no longer felt safe or comfortable in the task force, but knowing that there was only generall one way out, he began forming an escape plan.
He slowly began piecing together equipment for him to use once he left, and accrewing favors that would allow him to initially get out of the Empire relativly unnoticed.
When the day came, Torias slipped out and stole aboard a freighter bound for the Middle Rim - he inteded to pay his respects to Lorgar's family and inform them of his unfortunate demise; in doing so he hoped to clear his consious.
Torias is very aware, however, that his desertion, and knowledge about Imperial goings on, would lead to him having an incredibly large bounty on his head, and most likely a target for Imperial assasins.
Motivation
Torias is motivated by the betrayal of his friends and companions when they, without hesitation, murdered one of their own in cold blood - simply on the whim of a command.