Severus Snape killed Albus Dumbledore because it was part of a prearranged plan between them. Dumbledore was already dying from a curse inflicted by a Horcrux ring, and he wanted to die on his own terms rather than prolong his suffering. He asked Snape to kill him to grant a dignified death and to prevent Draco Malfoy, who had been assigned by Voldemort to kill Dumbledore, from having to commit the murder and damage his soul. Additionally, Snape's act was crucial to cementing his loyalty and trust with Voldemort, allowing him to continue working as a double agent to protect Hogwarts and help Harry Potter. The Killing Curse from Snape also ensured Voldemort would trust Snape, positioning Snape strategically to help the "good side" from within the Death Eaters. Dumbledore’s death served multiple purposes: sparing Draco, securing Snape’s role, and controlling the power dynamics involving the Elder Wand.