The Riddler is one of Batman's most infamous foes. He has been seen in many iterations since his introduction in Detective Comics #140 by Bill Finger, Dick Sprang, and Charles Paris back in 1948. However, some forget how truly vicious he can be. His sadistic nature is most clearly seen in "Dark Knight, Dark City", a Batman storyline by Peter Milligan and Kieron Dwyer.
The story shows the Riddler at his most bloodthirsty and is a bit of a departure from his usually corny riddle-based crimes. He originated as a more playful character, but over time he became darker -- like many of Batman's stories have -- and a more sinister figure and becoming even more sadistic than the Joker.
During "Dark Knight, Dark City", the Riddler uncovers a secret ritual called "the Ceremony of the Bat" and attempts to lure Batman into a trap to unleash a supernatural entity called Barbathos to destroy Gotham. As part of his scheme, he kidnaps four babies and puts them in mortal danger as part of his clues. This includes lodging a ping pong ball into one of the baby's throats which forces Batman to perform a traumatizing tracheotomy to save the child. That is incredibly grizzly, sadistic behavior. He is so specific in his violence which elevates the sadism beyond mindless chaos -- it's personal.
Even one of the Riddler's own henchmen is shocked by his sadism and remarks that the Riddler is, “starting to make the Joker seem positively sensible." The Riddler's behavior seems even more sadistic than Joker's because Riddler's actions are just as sporadic and senselessly violent, yet he maintains his patterns of clues and riddles making all of his actions premeditated and not senseless chaos but cold and calculating destruction. While his actions seem sporadic, they are deeply personal because he selects his victims to create the maximum traumatic impact versus a truly random selection process.
Part of what makes this version of the Riddler so terrifying is that Batman does not outwit him or trap him. The Riddler ends up trapping Batman and nearly sacrifices him to Barbathos. He is only stopped because he is terrified by Barbathos' confession that he was behind Batman's creation. Ultimately, Riddler is defeated by his own scheme turning sour on him instead of being outwitted.
"The Dark Knight, Dark City" arc elevated the Riddler, much like "Hush" did, in which he teams up with Dr. Thomas Elliot, aka Hush, to torment Bruce Wayne. Riddler learns that Bruce Wayne is Batman, and that Jason Todd was Robin. Riddler uses this information to torture Bruce who still carries tremendous guilt over Jason's murder waging extreme psychological warfare. The Riddler uses his intimate knowledge to inflict pain, not for any particular reason other than to torment him. After the Riddler's failed attempt to threaten to expose Batman's true identity, he defaults to prying on Batman's guilt over the death of Jason. While it was the Joker who killed Jason, the Riddler ramped up that traumatic event to the next level and actually took advantage of the aftermath instead of living in a single moment.
Most underestimate the true villainy and sadism of the Riddler -- he is more than a goofy green suit and a few tricky riddles. "Dark Knight, Dark City" proved that he would steep to incredible lows to inflict pain and torment on innocent civilians and Batman -- including harming babies. His sadism is particularly pronounced when you consider that his riddles and clues require a specific and detailed level of premeditation and planning that is more personal and sadistic than mindless chaos. He is a character obsessed with his own brilliance and uses that to inflict a maximum level of pain and suffering.