I think a truly good piece of media is one where even if you have things spoiled you can still get a reaction. All things considered the main character pretty much always wins/lives and the main villain pretty much always loses/dies. And one I've noticed a lot is that the first even possible romantic interest is most likely who they'll end up with if they end up with anyone. It's not foolproof but I'd put a bit of money on it. It's the in between that matters.

To go a step further, sure he killed Dumbledore but without knowing why, and without knowing the real why that's revealed later, only a small part of the fun is lost. That's what makes it re-watchable to me. I'll remember the twists and the endings, but the emotions and the minutae are what keep me watching.

Can you describe what you found funny at the time? I'm genuinely curious what motivates behavior like this.

The funnier version was people doing it during the next book's release.

My wife and I have decided there is one thing that it's universally okay for us to spoil to one another in a book, movie, etc.: if the cat/dog survives till the end.

A mild reminder that humans tend towards inflicting pain on their fellow humans when there are no consequences.