The rivalry between Cersei Lannister (Lena Headey) and her younger brother Tyrion (Peter Dinklage) makes for one of the most compelling character dynamics throughout all of Game of Thrones. Cersei has hated Tyrion since the moment he was born, since she blames him for "killing" their mother, Joanna Lannister, who died in childbirth.
This hatred grows further when Cersei accuses Tyrion of murdering her son/his nephew, King Joffrey Baratheon (Jack Gleeson), and when Tyrion slays their father, Tywin Lannister (Charles Dance), on his way out of Westeros. Cersei's loathing of her brother in the wake of Tywin's death makes for a high point in A Feast for Crows, the fourth book in Game of Thrones' source material, George R. R. Martin's A Song of Ice and Fire, when she becomes so paranoid that Tyrion, after murdering both his parents, will kill her next that she suspects he's hiding out in the walls of the Red Keep.
However, Cersei herself commits a similar atrocity to Tyrion later in the show's run. Whether she realizes it or not, Cersei is also guilty of the very same crime she's hated Tyrion for throughout his entire life.
The destruction of the Sept of Baelor
In the Game of Thrones season 6 finale, "The Winds of Winter," Cersei is set to be tried by the Faith of the Seven at the Great Sept of Baelor. However, Margaery Tyrell (Natalie Dormer) notes that Cersei is absent, despite understanding the consequences of missing her trial. As such, Margaery suspects that Cersei has something up her sleeve, though her concerns are dismissed by the High Sparrow (Jonathan Pryce).
As it turns out, Margaery's suspicions proved to be correct, as Cersei had secretly been scheming. She reignited the caches of wildfire planted by the Mad King, Aerys Targaryen, and blew up the Sept, killing several of her enemies in one fell swoop. Her victims included the High Sparrow, Margaery Tyrell, Margaery's brother Loras (Finn Jones), Margaery's father Mace (Roger Ashton-Griffiths), and Grand Maester Pycelle (Julian Glover), who she lures away from the Sept shortly before its destruction, only for Qyburn (Anton Lesser) to facilitate his murder at the hands of several "little birds."
Sadly, Cersei's last surviving child, King Tommen Baratheon (Dean-Charles Chapman), witnesses the Sept's destruction from his chambers in the Red Keep. Knowing this means his beloved wife Margaery has died, he wordlessly removes his crown and jumps from the window, killing himself. A mournful Cersei then becomes Queen of the Seven Kingdoms, ascending to the throne relatively unopposed given that many of her most notable enemies have all been killed.
The deaths of Lancel and Kevan Lannister
In addition to the Tyrells and the High Sparrow, Cersei's explosion also kills two other notable characters throughout the series: her uncle, Kevan Lannister (Ian Gelder), and his son, Lancel (Eugene Simon). Lancel's death is especially important, as he played a role in several of Cersei's crimes: He helped her provide her late husband, King Robert Baratheon (Mark Addy), with too much wine on a hunting trip, resulting in his death at the hands of a wild boar, and the two slept together shortly thereafter, engaging in yet another incestuous relationship. In fact, it was Lancel who reported Cersei's misdeeds to the High Sparrow in the first place.
As such, Cersei, after killing her uncle and cousin, has become a kinslayer. Kinslaying is often cited as among the worst crimes throughout the Seven Kingdoms, a warning that Rickard Karstark (John Stahl) issues to Robb Stark (Richard Madden) shortly before his execution, claiming that Starks and Karstarks share blood. Sure enough, Robb indeed finds himself cursed not long thereafter, as he is slain, alongside his mother, wife, direwolf, and several of his men, at the infamous Red Wedding.
Cersei becoming a kinslayer is especially prominent, as this is the crux of her hatred of Tyrion. Cersei loathes Tyrion for his role in the deaths of Joanna, Joffrey, and Tywin, although the audience knows Tyrion cannot be blamed for two of these. Cersei herself has now killed two Lannisters, more than Tyrion textually has.
To make matters worse for Cersei, she plays a larger role in Tommen's death than Tyrion plays in Joffrey's. As viewers eventually learn, Joffrey's murder was orchestrated by Olenna Tyrell (Diana Rigg), Margaery's grandmother, who wanted her granddaughter to marry the gentler Tommen instead of his cruel older brother, as well as Petyr "Littlefinger" Baelish (Aidan Gillen), who needed a distraction to sneak Sansa Stark (Sophie Turner) out of King's Landing.
Tyrion was simply an unwitting fall guy amid this scheme. Cersei, meanwhile, actively killed Margaery, which resulted in Tommen's suicide. An argument can be made that she has played a role in the deaths of three different Lannisters, making her far worse of a kinslayer than Tyrion, further emphasizing how irrational her hatred of him has become.
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