After the Duros Stations were effectively enslaved by the empire following the declaration of the Rebellion in 2bby, he sought out and joined the rebellion. If you met him now you would think him a truly empathetic person who wants to end the horrors of Imperial slavery and who cares deeply about his men. However, that’s not how this rebel got started.
Sōn Kinsen joined up purely for revenge for the death of his twin sister, Alī. She was peacefully protesting the empire's forceful impressment of Duros political prisoners into slave labor. Alī was gunned down by the imperial garrison after a single bottle was thrown at the stormtroopers. Sōn was 25 at this point and was working for meager pay on an imperial factory assembly line. Before this Sōn and Alī were rough and tumble kids, running around a station filled with poverty, getting into trouble. Neither of them had many prospects for the future. Sōn would get into fights with the other kids at school. Sometimes it was for fun, but most times he would be fighting to protect his sister. She was very bookish and was thought of as a gifted child by the teachers. While Alī was able to go to a good university, Sōn started working at the same assembly line where he would later hear word of the dead protesters and his dead sister.
This massacre and other earlier atrocities had caused a mass exodus of Duros people from the stations and many of them, including Sōn, went to the rebellion. Sōn was empty after the death of Alī. He felt that the only way to fill that emptiness was to kill the enemy. After he was trained to become a rebel soldier, Alī always tried to get assignments where he could directly take on the empire. His reputation became one of a hardened, angry warrior who showed no mercy. This behavior would have been a redflag for the rebel high command but to give one soldier out of thousands disciplinary action when they were stretched thin would have been a misallocation of resources. It wasn’t until Sōn was given a mission that returned him to the Duros stations that he gained a new found purpose.