Over the 6 seasons of Supergirl, Kara Danvers has gone through a great deal of evolution. Considered the symbol of hope in the CW's Arrowverse, it's hard to imagine the daughter of Krypton ever acting like a villain. Yet she has displayed some antagonistic characteristics on numerous occasions.
Fans have to credit Melissa Benoist for being able to portray so many different sides to Kara. While her evil turns might not always be her fault, or perhaps might not even be the Kara Zor-El of Earth-Prime, it's still quite unnerving seeing Kara act in such a malicious manner. Regardless, at some stage, all of DC's heroes have to visit their dark side before truly coming to represent the light.
Honorable Mention: Not Using Her Powers
While not acting outright evil, it's still pretty bizarre that Kara would go the majority of her young adult life without using her powers. While audiences get to see her finally forced to make a decision, to save a crashing plane, there would have been plenty of moments when Kara looked the other way.
Clark Kent went through the same dilemma when deciding whether he should show the world his gifts, but Supergirl had seen the impact of Superman and still decided she was best hidden in the shadows. To think about how many lives Kara might have saved is to really confront the fact that she may have been responsible for unnecessary losses.
Red Kryptonite
While she never meant to betray her team, Kara was a significant and evil threat to her friends after being infected with Red Kryptonite. There are various versions of the substance, with each color actually having a different impact on a Kryptonian.
The Red Kryptonite fills its victim with rage. Supergirl couldn't resist the impact of the rock, ultimately coming close to killing those around her. Its effects would eventually wear off but it was certainly a scary moment to see such a powerful being lose complete control.
Lacing The Atmosphere With Lead
Daxam and Krypton had been rivals for years and clearly for good reason. Despite her alliance and romance with the Daxomite Mon-El, his people came to attack Earth. For Supergirl, there appeared to be only one way to protect the planet she had come to call home.
Lead is incredibly dangerous for Daxomites. Enough of it could kill them. To stop their assault Kara, therefore, had the atmosphere laced with lead. Not only could this have killed thousands, but it also meant that Mon-El had to leave the planet too. There were surely other decisions to be made in the situation but Kara went for one that sounded like a villainous master plan.
Earth-X Kara
While it's not the Kara of Earth-1 or Earth-Prime that audiences are used to, there was a Kara from another reality that actually sided with the imperialistic, Nazi regime that had been formed. Known as Earth-X, heroes like The Flash, Green Arrow, and Supergirl had all morphed into something more dangerous.
Named Overgirl, the character helped to carry out the Nazi's plans after winning World War II. While she was eventually defeated and the story was not revisited again, it goes to show how lethal Kara can be if her past was shifted by world events.
Manipulating Lena
Kara knew that so many people had lied to Lena Luthor to the extent that the most intelligent member of Team Supergirl barely trusted anyone anymore. She continued to hide the truth of her secret identity from her best friend, which ultimately caused a rift between the pair.
Kara manipulated Lena and almost turned her into a fully-fledged foe. She vilified her former teammate and even put a stop to some of Lena's heroic efforts. It's easy to see both sides of the conflict but Kara didn't exactly come out looking heroic from their struggle.
Red Daughter
The Red Son storyline is one of Superman's most famous narrative arcs from the comics and it's a plot thread which the CW allowed to play out in Supergirl. With Kara's background instead influenced by the Soviet Union, she comes to learn to hate the United States.
Again, this might not be the original recipe Kara but it's another Elseworlds narrative that demonstrates what would happen if Earth-1's Kara was put in a similar situation. Ultimately, Red Daughter had enough of Kara's optimism and hoped that she began to reject the narrative she had been fed.
Changing Reality
It may be one of the show's least realistic storylines but much like a villain would, Kara enlisted the help of Mr. Mxyzptlk to try and right the wrongs of her past. Offering to change her reality, Mxy showed Kara a series of potential timelines where she acted differently in each.
The idea was to be able to mend the bond with Lena, but there was ultimately no way to do so. In getting Mxy's help, not only did Supergirl mess around with the universe for her own benefit, but she also sought to manipulate the lives of her friends and opened herself up to their secrets without giving them any privacy.
The Dark Days
While it was heartwarming to see Kara's life finally restored and even to witness the reunion of Supergirl and Mon-El, there were some dark days in between. After sending Mon-El away, Kara went into a much darker frame of mind, almost acting like a villain and shutting down her human side.
She shunned her friends and family, took a more violent approach to her duties, and essentially shut down Kara's life to only be Supergirl. Her personality became very similar to the grimmest versions of the Caped Crusader and felt as if Kara was about to fall over the edge and go evil for good.
Early Violence
In Kara's earliest years as Supergirl, she fought with some anger and a lot of naivety. Without people like the Martian Manhunter and her sister Alex around her to reign her in, it's unclear how Supergirl would have evolved over time. Her team are her absolute rock.
There were a number of occasions where Kara perhaps took her justice a bit too far. A traffic collision that caused an argument for instance saw Kara significantly injure a man's arm in front of kids, which led to a backlash against the hero. Small moments of villainy like these could have eventually spiraled into Kara creating her own justice in the field; a path that many evil-doers have taken.