It’s not “once a Death Eater, always a Death Eater.”
He may have been weird as a kid, but Snape actually became straight-up evil for a while there. One might have even thought him among “the vilest of sinners.” After Hogwarts, he joined the ranks of the Death Eaters and became one of Voldemort’s most trusted servants. In fact, by relaying the contents of Sybil Trelawney’s prophesy to the Dark Lord, Snape was indirectly responsible for the murder of James and Lily Potter.
But though his sins were crimson red, Snape eventually saw the light. He spent decades as a spy for Albus Dumbledore, masquerading as a Death Eater and risking his life at every turn. Many in the Order of the Phoenix questioned his loyalty—“No one stops being a Death Eater,” said Sirius—but ultimately, Snape never wavered. His repentance proved absolutely critical to Voldemort’s demise.
Don’t judge the Slytherins too harshly—sometimes they’re the best among us.
“If the Sorting Hat had tried to put me in Slytherin,” said Ron Weasley, “I’d have got the train straight back home.” This attitude toward Slytherin house was not uncommon in the wizarding world. Although they were selected for their cunning, resourcefulness, leadership, and determination, Slytherin students had a bad—though perhaps partly deserved—reputation. As Ron explained, “There’s not a witch or wizard who went bad who wasn’t in Slytherin.”
For most of the series, readers had no reason to believe that Snape was good other than Dumbledore’s unexplained trust. And why would they? Snape was mean, vindictive, and perhaps most foreboding of all, the head of Slytherin house. But his late-game heroics proved a powerful point—bravery and value are often found in the unlikeliest of places. Readers never judged Snape correctly because they didn't know his whole story.
The same is true of us today. How often do we judge others only to find out we were mistaken all along? Or, even if our assumptions were correct, how often do we learn something about them that gives us more insight into their struggles? Sometmes the most difficult people to love or trust are the ones who most need it.