Why Does Sansa Have Littlefinger Killed?
This Is Exactly Why Sansa Makes Her Final Decision About Littlefinger
Jon Arryn's death
This one encompasses a lot of things. First of all, Littlefinger always loved Sansa and Arya's mother, Catelyn, going back to when he fostered with the Tullys as a boy after the death of his father. He always loved Catelyn and her sister Lysa always loved Littlefinger. So he used this affection Lysa had for him to begin his machinations for power (and possibly Catelyn's heart) by helping Lysa kill her husband, Jon Arryn, hand to King Robert Baratheon, and pinning it on the Lannisters.
Littlefinger, who is usually three steps ahead of everyone else, had to have known that Ned Stark would leave Winterfell to investigate these claims, so it put him a little closer to ending the Stark marriage and possibly taking Catelyn for his own. It also turns the Starks and Lannisters against each other, something that definitely benefits someone who thrives on chaos and mistrust.
