Original: When Voldemort kills Snape later in the story, Harry realises that Snape was not the traitorous murderer he believed him to be, but a tragic anti-hero who was loyal to Dumbledore.
Simplified: When Severus Snape is killed by Voldemort later in the story, Harry realises that Snape was not the traitorous murderer he believed him to be, but a tragic anti-hero who was loyal to Albus Dumbledore.