As strange as it might be to read, Deathstroke once believed in a cause. A look into his past demonstrated how loyal he was to the United States military back when he was a career soldier. However, Deathstroke Inc. #10 (by Ed Brisson, Dexter Soy, Veronica Gandini, and Steve Wands) made it clear that his treatment during the experiment that made him a super-soldier changed his views on serving a country.
The way the military casually discarded him after the experiment appeared a failure made him bitter. He felt betrayed by the country he had given everything. As a consequence, he would not and could not forgive that. That sting of betrayal may explain why Deathstroke took up mercenary work instead of returning to active duty with his new powers: It had shattered his belief in fighting for anything but himself.
The issue revealed that the same experiment that made Deathstroke the threat he is in the present also killed several other soldiers. Slade Wilson was an oddity amongst the test subjects because he survived the ordeal. Despite that, the military still deemed him a failure due to his weakened state. Unbeknownst to them, he could hear all of their assessments. He angrily realized that after over a decade of service, the very people he had sworn his life to serve had turned on him because of events outside his control.
What makes this even worse is that Slade had volunteered for this experiment. No one had forced him into it. There was no personal pressure. He chose this. Back then, Slade was a patriot. He thought this experiment was a chance to serve his country better. Instead, all he got from it was months of suffering and the early termination of what should have been a distinguished career. That would have made any man upset.
However, Slade wasn’t like any other man, especially after the experiment. The doctors treating him had jumped the gun and falsely assumed that their experiment had yielded no results. This allowed Deathstroke to adapt to being a super soldier in private, but he never forgot how easily the people that created him discarded him.
This might explain why, once he had adjusted to the change and had a better grasp on what he was capable of, he didn’t return to being a soldier. He could have been an invaluable asset to the military, but by that point, he had realized that was all he could ever be: an asset. A tool for them to use and cast aside when it was convenient. The military lifestyle no longer appealed to him because he saw that there was no reward for his loyalty.
If his devotion was not to be appreciated by the world’s most well-funded military, whose soldiers are supposed to be valued, then he saw no reason to devote himself to any other cause than his own. This critical mistake on the military’s part created a monster who would fight for the pettiest of reasons and one bound not by morals but selfishness.