The short version is brutal and tragic: Severus Snape is murdered by Voldemort's snake, Nagini, in 'Harry Potter and the Deathly Hallows'. Voldemort orders the attack because he believes that killing Snape will make him the true master of the Elder Wand — he thinks Snape killed Dumbledore and thus holds the wand's allegiance. Nagini lunges and fatally wounds Snape; he dies shortly after, leaving Harry with his last, crucial memories.
Reading that sequence in the book felt like being handed the last pieces of a giant, heartbreaking puzzle. After Snape's death Harry takes his memories, uses the Pensieve, and learns the full, double-edged truth about Snape's loyalties, his love for Lily, and all the sacrifices he made. The films follow the same essential beats — Nagini kills Snape on Voldemort's orders, Harry receives the memories — but the movies compress and stage the scene differently, focusing on visual emotion rather than all the internal detail the book offers. For me, both versions land hard, but the book's unspooled memories give Snape a painfully clear human finish that stuck with me long after I closed 'Harry Potter and the Deathly Hallows'.