I stumbled by accident on the ending while trying to find a walkthough (I was looking for gameplay videos)

Should I still buy it even If i know that? It seemed like a twist to me....

Even if you know the ending, get it for the riddles.
It is a great game

There's also more than one endings, so if you haven't learned the other ones, it's worth it to find the different ones. Plus, it is all about the story, not the ending.