From the reviews and comments and stuff I saw I was under the impression, that that was kind of the whole point of their arc, that they had been vigilantes so long that they had changed and gone against what they originally were when they first started. I got the impression that they were kind of a cautionary example for what Robin might become if he continued down the darker path he was on.
Except the show establishes that they were always violent, with a flashback revealing that Batman was telling Robin to stay away from them years ago because of how violent they were. They didn't become violent, they've been violent since they first started being vigilantes.
Plus, Robin murders more people in the first two episodes then they do in one (unless you believe Robin sticking large chunks of metal through people's throats to be non-lethal, like some of the show's fans would probably claim), so he definitely isn't less dark or violent then they are.