Robin Hood: Prince of Thieves

Robin of Locksley (Kevin Costner), an English nobleman who joined Richard the Lionheart in the Third Crusade, is captured and imprisoned in Jerusalem along with his comrade Peter. Robin engineers an escape, saving the life of a Moor, Azeem (Morgan Freeman) in the process; Peter dies in the attempt and has Robin swear to protect his sister Marian (Mary Elizabeth Mastrantonio). Robin returns to England with Azeem, who vows to accompany Robin until the debt of saving his life is repaid.In England, with King Richard away, the cruel Sheriff of Nottingham (Alan Rickman) rules over the land, aided by his cousin, Guy of Gisbourne (Michael Wincott) along with the witch Mortianna (Geraldine McEwan) and the corrupt Bishop of Hereford (Harold Innocent). At Locksley castle, Robin's father (Brian Blessed) is lured to the gates and captured by the Sheriff's men after refusing to join them.Robin returns to England to find his father dead, his home in ruins, and the Sheriff and his men oppressing the people. On his flight from the Sheriff's forces, he and Azeem meet with a band of outlaws hiding in the Sherwood Forest, led by Little John (Nick Brimble). Also among them is Will Scarlet (Christian Slater), who is later revealed to be Robin's illegitimate half-brother. Robin eventually assumes command of the band, encourages his men to fight against Nottingham and trains them to defend themselves. They begin to rob English soldiers and convoys that pass through the forest, then distribute the stolen wealth among the poor. One of their early victims is Friar Tuck (Michael McShane), who subsequently also joins the Merry Men. Robins successes infuriate the sheriff, who increases the maltreatment of his people, resulting in more support for Robin Hood despite the Sheriff's attempts to defame him and ever-growing rewards on his head.Finally, the sheriff has had enough. Hiring Celtic warriors to bolster his forces, he tracks down the outlaws' hideout and initiates a massive attack which destroys the forest refuge. He also restrains and confines Marian when she tries to summon help from France. The sheriff proposes to Maid Marian, saying that, if she accepts, he will spare the lives of the captured woodsmen and their families. Nevertheless, several of the rebel fighters including Little John's son are to be executed by hanging.However, despite information to the contrary, Robin and a handful of his most trusted aides did survive the assault. On the day of the sheriff and Maid Marian wedding and the scheduled hangings, Robin and his men fight their way into Nottingham castle and free the prisoners. The original plan was to just free their friends and retreat, but then Azeem reveals himself and his willingness to fight the sheriff, finally turning the peasants to revolt. Robin kills the sheriff, avenging his father. With his guard down, Robin is attacked by Mortianna, who charges with a spear. Azeem throws his sword, slaying Mortianna and fulfilling his vow.Robin and Marian profess their love for one another and marry in the forest. Their wedding is briefly interrupted by the return of King Richard I (Sean Connery), who blesses the marriage and thanks Robin for his deeds.

Question: Why does Robin kill the sheriff?
Answer: To avenge his father's death.