Well, Batman has info containing all of the weaknesses of the Justice League, and perfectly planned out strategies to take them down if they ever go rogue. I would assume Robin has done the same with his team, and if he turned evil, he would use those to his advantage.
Robin was trained by both Batman and Slade,the former for majority of his strengths,and the latter training him to attack with less hesitation,even towards those whom he loved. Robin is also the leader of the Teen Titans,and 1. the Titans would have a very hard time overcoming their will to not attack another team member,and 2. without their leader,they would be rather unorganized unless another stepped up to the role.
Cyborg is half robotic and could thus probably withstand more attacks than the shorter and smaller Robin. Cyborg would also have more technology and knowledge of the Titans' attacks and weaknesses,and if the battle were to start in the Titans' Tower,Cyborg could easily configure every trap in the Tower to attack them easily.
Robin is the former sidekick of Batman, the one who had been Gotham City's protecter for a long time and was trained by him. He knows the Titans secrets and weakness inorder to beat them, is stronger than any average human, and is the leader of a superhero group depite being only human. While the TTG version seems to be emotionally unstable he still does keep the team in good terms despite his problems. With him as a villain will be like a nightmare to the other Titans.
Raven is a daughter of an evil, intergalactic demon who has conquered many dimensions. While she does choose to be good, just one small snap, she could practically destroy the city with her demon powers as seen in Dog Hand and Nose Mouth, especially when she becomes truly demonic. You guys saw her dream in Dreams, that could happen any time at all when she's evil!
Starfire might be sweet and all but she's totally scary when she wants to be. I mean you guys saw Starfire the Terrible! She makes a very evil villain. With her temper and also with the fact she's one of the most powerful Titans being evil is no problem at all for her especially when she doesn't get what she want.
Cyborg is a half human and half robot. So he knows the weakness of an average human and superhero. Even though he is goofy at times he's serious at any time he needs to be. Also he can build an army of robots at a day, complete with any kind of weapons. Evil is something he can do.
Beast Boy is a hero... who... lacks common sense and intelligence, but doesn't mean he can't be evil. His animal insticts reacts quickly at any time. If he doesn't control all of them things will get very messy. As seen in Gorilla he uses his animal strenght to simply get what he wants or for others to follow his rules instead of the right rules. If Robin had not stop him sooner the whole city will be in total chaos the way he actually likes it and thats where the evil will kick in.
I think all of the Titans are very good heros, but that doesn't mean they can't be evil villains too