Background
The folks over at Baktoid had a crazy idea. What if we took one of our most advanced combat droid, removed the inhibitors, and granted them the freedom to do whatever they want, could we make a droid that was more effective and still loyal to their creators/buyers?
No, the answer's no. That didn't stop them from trying though. R0-9 is one of the many droids that was a result of that experiment. Coming off the assembly line he had no military ambitions, and as soon as it was clear he wasn't required to be there he left.
While combat didn't appeal to Ronin, combat sports did. With a few new friends Ronin started an underground droid gladiator ring, where he spent his days smashing other droids and reaping the profits. Unfortunately, while the Republic doesn't see droids as people they did decide it would be "inhumane" to allow them to "kill" each other for entertainment, so the operation was shut down. Ronin's business partners were arrested but he was set free, being a droid they assumed he was a victim of the operation instead of recognizing him as its ringleader. Ever since that event Ronin has despised the Republic.
Motivation