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.
By his own admission, Tyrion is a monster to most of the people in Westeros - but nobody feels that way about him more than his elder sister, Cersei. Their mother passed while giving birth to Tyrion, a fact Cersei will not forget. While their brother Jaime had a soft spot for Tyrion, Cersei did everything she could to demean and insult Tyrion's character.
Part of Cersei's hatred towards Tyrion comes from a prophecy she learned from a fortune teller when she was a child. She was told the valonqar would strangle the life from her and, because "valonqar" is High Valyrian for "little brother," she likely believes Tyrion may one day come after herself as well.
The constant mistreatment from Cersei clearly took a toll on the youngest Lannister child. Tyrion became something of an outcast from the family and, when Joffrey was slain, he ultimately severed his ties with the rest of the Lannisters. He was then captured and taken to become the Hand of the Queen to Cersei's greatest rival, Daenerys Targaryen.
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.
Ellaria Sand never got over the passing of Oberyn Martell, and she blamed Cersei and the rest of the Lannisters for his demise. Ellaria took out Cersei's daughter, Myrcella, via poison, while Jaime, the girl's true father, helplessly watched her perish.
Cersei decided the best way to handle that sort of anguish was to inflict it right back onto Ellaria in the same manner. She requested Euron Greyjoy capture Ellaria and her daughter, Tyene. Both were brought to King's Landing and imprisoned within the Red Keep. After sentencing Ellaria to life imprisonment within the Black Cells, Cersei forced Ellaria to watch as she offed Tyene with the substance used to take her own daughter's life.
When Tyrion was accused of slaying King Joffrey, he was taken to a trial where nobleman after nobleman testified against him. These people were likely bribed or coerced in some way by Cersei and Tywin, but the one person whose testimony stung the most was definitely Shae's. Tyrion had come to love his consort, and he believed she loved him as well, which is why her lies on the stand were so agonizing.
It's unlikely Shae would have testified against Tyrion had Cersei not coerced her in some way.