The Vampire Diaries' infamous Salvatore brothers were in constant debate through the entire series; who's the nicer one? Who had a better relationship with Elena? Who could be more evil? In terms of "evil-ness," both Damon and Stefan had manipulator tendencies but without their humanity, they were both ten times worse.
Stefan and his "Ripper" persona were always just one horrendous day apart, constantly playing a tug-of-war. Although he's known as the usual example of a strong moral compass, especially in comparison to Damon, everyone slips and when Stefan does, it goes south very quickly.
Stefan's 10 Year Killing Spree
One of the worst things to happen to on The Vampire Diaries and to Stefan, Stefan's 10-year spree of leaving bodies piled along the Eastern seaboard was vicious and gory. Within these 10 years is when he first met Klaus and Rebekah in the '20s and those two never bring along any good.
Fans quickly learn how much Stefan's "Ripper" alter-ego lives are a tremendous weight on his shoulders the entire season, It's one of his core qualities actually. While Lexi was the best friend to always pull him back, eventually Stefan doesn't have her anymore.
Stefan Made A Human Drink Blood
1920s Ripper Stefan is him at his pinnacle of atrociousness. Having Rebekah as a love interest led him to meet Klaus and the two quickly became friends (brothers even). While reminiscing in Season 3, Klaus reminds Stefan of his "favorite trick" that he learned from him in the '20s.
After feeding on a woman for the night, her husband came to confront Klaus and Stefan. This altercation led to the husband being compelled to sit and drink a glass of his wife's fresh, warm blood.
Stefan's List-Keeping
Known to be a guilt-ridden individual overall, Stefan's Ripper days are guaranteed to torment him for years after he returns to humanity. His bloodlust and his lack of control are what led him to his infamous "bunny diet."
One key characteristic of Stefan has always been his compassion, even in his darkest days. During his "Ripper" stint in Chicago during the '20s, Stefan kept a list of every person he murdered written on a wall in his closet. While Klaus believes it's for the sake of "reliving the kill," Damon believes it's for his own conscience, fueled by regret.
Stefan Spent Most of Season 3 Being Rude To Elena
In an attempt to save Damon from a werewolf bite, Stefan willingly surrenders himself and eventually his humanity to the hands of Klaus Mikaelson, the Original vampire, in season 3. Stefan's vampirism and the amount of control he can or can't have on it at any given moment was a core part of his arc and one of the reasons Stefan was The Vampire Diaries' main character.
After resisting Klaus' compulsion to harm Elena, that's when the famous Original had him shut off his humanity for the first time since being with Elena and he spent practically the entire season being completely unbearable. Resorting back to human bodies to feed, Stefan even goes to call Elena a "human blood bag."
When Stefan Threatened To Kill Elena
While it isn't 100% clear if Stefan had his humanity on or not on in this scene because the season 3 switch back on is murky, it still exists as one of Stefan's most villainous moments in The Vampire Diaries. In a revenge plan against Klaus that involved stealing his family's coffins, Stefan takes Elena to Wickery Bridge (where her parents died) and threatens to kill her if Klaus doesn't remove his hybrids from Mystic Falls.
To make matters worse, Stefan forced Elena to drink his blood, making the threat now turning the human doppelgänger into a vampire. Elena had so much trauma with this bridge that Stefan doing that whole debacle made it so abominable.
When Stefan Went With Caroline On A Killing Spree
Stefan without his humanity is a force to be reckoned with, so when Caroline turns hers off after the death of her mother and then forces Stefan to shut his off, the dynamic duo turn treacherous. Both vampires go on a killing spree and the "Ripper" comes alive.
Stefan's humanity only gets triggered on by him reuniting with his mother Lily, and, in turn, helps Caroline come back from the edge by showing her one of his last memories with her late mother.
When Stefan Killed Enzo
Stefan murdering Enzo with his humanity off is not only one of the worst things he's done, but it's specifically one of the worst things he does to Bonnie in the series. In an attempt to kill Elena in her coffin and free him and Damon from Cade's servitude in season 8, he ruthlessly rips Enzo's heart out, right in front of Bonnie's eyes.
The heinous act can easily be looked at as circumstantial with Stefan having no humanity and all, but it immensely turns pivotal when Bonnie strikes him with the vampire cure, turning the main vampire of the story human again.