"There was no difference between what I was doing and what the villains were doing."

In Mixed Doubles, Doyle says the following about his reasons for leaving the police and joining CI5:

"The line was getting too narrow, what I was doing. There was no difference between what I was doing and what the villains were doing. In the blue corner--right; in the red corner--wrong."

What does this actually mean? What was he doing in the police that he doesn't do in CI5?

I would think that CI5 has more in common with the villains, given its "by any means necessary" brief. We see Bodie and Doyle threatening people with various kinds of harm, breaking into houses and cars, smashing up a restaurant, press-ganging a random (drug-carrying) guy into participating in one of Cowley's schemes, stealing fruit. The police, at least in theory, have limits on their behavior. I realize those were pretty elastic at the time, but they do exist in the world of the show, else there would be no point in creating an organization without such limits.

So, what do you make of that piece of dialog?