why did they kill joel in last of us 2

Joel was killed in The Last of Us Part II because he had killed Abby's father, who was the lead surgeon of the Fireflies—a rebel group trying to find a cure for the fungal outbreak by using Ellie, who is immune. Joel's killing of Abby's father was a pivotal and controversial act from the first game, as Joel saved Ellie but thwarted the Fireflies' plans. Abby and her group sought revenge on Joel for this act, leading to Joel's brutal murder, witnessed by Ellie, which serves as the inciting incident for the story of The Last of Us Part II. The showrunners of the HBO adaptation kept Joel's death early in the series to align with the game's story and to drive the narrative of revenge and consequences forward. Joel's death is meant to be a senseless act of violence that motivates the rest of the story, especially Ellie's quest for vengeance.