Anyone who has read the books or watched HBO's Game of Thrones knows Cersei Lannister is a truly evil woman. Granted, everything she does is for a reason, and she probably doesn't see her actions as being evil, but to everyone else who suffered at her hands over the years, it's hard to argue she isn't a cruel psychopath. As such, she is one of the show's most hated characters, but due to her brilliant portrayal by Lena Headey, fans also love watching her.
Of the many acts that define Cersei as a truly evil character, some stand out as her worst moments. Since she managed to survive from the pilot episode into the eighth and final season, there's a lot to go through. She's done some horrific things throughout her time in Westeros in her quest for the Iron Throne.
When Joffrey playfully mistreated Arya's direwolf, the creature unsurprisingly bit him. He had it coming, but as he was the eldest son of the king, something had to be done in the name of justice. Arya's direwolf had run away, which left no means of righting the wrong. Typically, they would have slain the animal and moved on, but since it was no longer available, Cersei demanded the next best thing: instead slay Sansa's direwolf, Lady, who had done no wrong whatsoever.
It was a cruel order and one Eddard Stark ultimately carried out. Initially, Ilyn Payne was directed to take care of Lady, but Stark insisted he be the one to carry out the execution, citing "she deserves better than a butcher."
When Cersei was held by the High Sparrow, she endured severe mistreatment and embarrassment that a woman of her standing would normally never be subject to. To escape the situation, she confessed to sleeping with her cousin, which led to her walk of atonement. Her humiliation only grew as she had to stroll unclothed through a city full of angry citizens throwing rotten fruit, mud, and feces at her. It's not surprising she found a way to get revenge.
When Cersei's son Tommen was set to marry Margaery Tyrell, the Great Sept of Baelor was full of people who had wronged her. She instructed Qyburn to ignite the barrels of wildfire beneath the sept, completely annihilating the building along with everyone inside. When Tommen saw what happened across the city, he immediately ended his own life.
Not only did Cersei wipe out a holy site in the capital city of Westeros, but she also slayed the leaders of the Faith of the Seven, as well as her son's fiancée, her own cousin, and countless others. Her actions led to the demise of her only remaining son.
Cersei is always looking for ways to make Tyrion's life worse, which means she isn't afraid to cast her gaze upon other people who associate with her little brother. When Tyrion ordered Myrcella to Dorne, he did so without consulting Cersei. Thus, Cersei wasn't pleased to have her only daughter taken to a faraway land, and she swore revenge against Tyrion.
Aware Tyrion was sleeping with someone, Cersei didn't know the identity of the woman. When Cersei saw Ros wearing a golden lion pendant Tyrion had given her in Winterfell, she had Ros captured and thrashed severely. However, Ros wasn't even the woman in question.
When Cersei was a girl, she was given a prophetic fortune that said she would remain queen until a younger, prettier woman came along to replace her. When Margaery Tyrell was first married to Joffrey, Cersei had suspicions. After Joffrey's slaying, the queen became paranoid. Margaery was then betrothed to Tommen, and Cersei's belief that Margaery would come to supplant her drove Cersei to near madness.
To remove portent, Cersei fabricated evidence against the young, would-be queen that implicated adultery. The High Sparrow took Margaery to answer to her transgressions. In the end, Margaery was able to negotiate her way out of the High Sparrow's suspicion, while Cersei herself ended up imprisoned in the same manner. Cersei got her comeuppance during this plot, but the wrathful vengeance that followed was one of her vilest acts of all.