Here's how Harry learns the truth:
* Snape's Memories: In the Pensieve, Harry witnesses Snape's memories. He sees Snape's childhood, his love for Lily, and the events leading up to Dumbledore's death. He sees Snape's unwavering loyalty to Dumbledore, even though it meant following his instructions, which included killing him.
* Snape's Sacrifice: Harry also learns that Snape was a double agent working for Dumbledore. He was protecting Harry from Voldemort and playing a critical role in the fight against him. He even used his Patronus charm to save Harry's life when Voldemort was after him.
* Dumbledore's Plan: Dumbledore knew his death was inevitable. He planned it all along. He knew Snape would be the one to carry out his plan, knowing that Snape's loyalty to him and his love for Lily would lead him to do what was necessary.
In the end, Harry realizes that Snape's actions, though seemingly terrible, were driven by love, loyalty, and a desire to protect Harry and the wizarding world. He realizes that Snape's life was dedicated to fighting Voldemort and protecting Lily's legacy.
The revelation of Snape's true character is a complex and emotional one. It highlights the complicated nature of good and evil, the power of love and sacrifice, and the tragic consequences of war.