Background
Mortimer is the third born of a minor Corellian dynasty brought up on the tales of the golden age of the Republic. Due to a proper upbringing, and the family business being entrusted to his older siblings as was tradition, he did the honorable thing expected of a man of his position and joined the Coresec police force. followed his family reputation of becoming a part of the prestigious Coresec police. All very typical of a high flying elite Core Worlder.
Then, the war came.
Given that experienced officers were in short supply the prestigious officer did the proper thing to volunteer his services, after all the corporations of the Rim had been a major opponent of republics right's movements for the last hundred years so the idea of bringing law to them would serve as a capstone for future endeavors in politics.
Of course, war would be nothing like the romanticised tales would make out.
Motivation
His primary motivation is to become a goddamn war hero. In true Corellian tradition, the only thing more important than wealth and independence is the strength of one individual's reputation, thus successfully defending the new republic and becoming famous would allow Dillenger to gain prestige within his society, along with making the family name more prestigious.
There's a certain amount of naivety behind this, however; he might have made a damn good cop, admit one that occasionally came to blow's with criminal organizations, but this civil war is probably much more than anyone could ever bargain for.