Defection, for a spy, is there anything more thrilling than turning an enemy's spy?
Although Brian is part of the action team, not a spy, and although Anton is not strictly speaking a Black Devil, he merely completed the training to join them.
But, but, but Anton indeed is the man closest to the Black Devils, and if one must say he is a Black Devil, then it wouldn't be entirely wrong.
The thing Brian, who harbored deep resentment against the Black Devils, most hoped for wasn't the complete annihilation of the Black Devils—of course, that was impossible—but if he could turn a Black Devil, then that could be said to be the greatest achievement of Brian's life.
Don't forget, it was the Black Devils who had turned Brian. This was the biggest stain of his life, as well as his greatest pain.
Therefore, Brian's thoughts were very easy to understand; he just wanted to save some face. He wanted to turn Anton, or rather, to take Anton into his own fold.