Background
To most, the Jedi and the Force are nothing but myths; stories to scare children into staying in line. Fewer still have heard of the Sith - and those who have are usually smart enough to discuss the subject in hushed tones, behind closed doors.
Fewer yet know of the Jensaarai - a Force-using group from the Suarbi system, separate from either Jedi or Sith. Initially founded by a fallen Jedi corrupted by an ancient Sith text, the Jensaarai have an idiosyncrestic mixture of teachings: incorporating Jedi ideals of peace, justice, and harmony, while advocating for a more direct approach at times. In a subversion of Jedi practices, a Jensaarai's most important step in transitioning from an Apprentice to a Defender was not the construction of a Lightsaber, but rather of their own suit of armor.
Many of the Jensaarai were killed when the Emperor found of their existence, although a large number still exist on Susevfi - hidden by the Force. Due to their previous interactions with the Empire, the Jensaarai dealt with Imperials harshly, torturing and executing any they found - including deserters from a planetary garrison.
Sevrahn was a descendant of one of the original Jensaarai, having never known another Chiss, or visited the planet Csilla. Thus, when the Jensaarai caught a Chiss Imperial Deserter by the name of Rynwen, Sevrahn was for the first time in the company of one of his own.
In her weeks of captivity, Rynwen told Sevrahn of the cruelty of the Empire, its oppressive destruction and of its hostile xenophobia. She described how she had defected from the Garrison in order to join the Rebel Alliance, and how she'd been in contact with a Sathari Rebel Cell Leader.
Unfortunately for him, he was also tasked with her torture and execution. When the time came, Sevrahn could not bring himself to carry out the grim task, and instead tried to help her escape.
She was caught.
Sevrahn was found guilty by his peers - but despite his treason to the Jensaarai, he was not executed. Instead, he was given the chance to live in exile, provided he never return to Susefvi.
Sevrahn complied, and without anything but his personal lightsaber, shield, and armor, left to make contact with the Rebellion...
Motivation