I feel like a lot of people get hung up on the surface-level 'good vs evil' duality thing and miss what Stevenson was really digging into. Sure, Hyde's the monster, but the ending's terror comes from Jekyll's total failure to control the division he created. He's not a victim of some external force; he's the architect of his own ruin. The 'freedom' he sought by splitting himself just ended up trapping him. The final chapters, written in his own voice, show a man utterly horrified by what he's become, but also weirdly fascinated. He's watching himself be erased. That's the chilling part – it's not a battle, it's a slow, inevitable surrender.
And honestly, the ending isn't just about Jekyll's personal collapse. Think about Utterson and Lanyon. The story closes with Utterson sitting alone with the documents, holding the truth but unable to ever really understand it. Society's polite, rational representative is left in permanent, unsettling doubt. The whole social order that Jekyll wanted to secretly bypass is shown as fragile, because the beast was inside the gentleman all along. The 'ending' is about the shattering of that Victorian ideal of respectability and self-control. It leaves you with this lingering question: if someone like Jekyll can harbor that, who's next? The door to his lab is broken down, but the mystery is only more disturbing.