School Days' ending leaves a bad taste in my mouth, because it was the product of popular demand. School Days is drawn from the visual novel of the same name, and of all the multiple plot lines and endings the studio could have gone with, they went with a mixed bag of the "Bad endings," all of which were infamously violent.
Visual Novels are essentially fully-illustrated "Choose Your Own Adventure books," usually with some romantic undercurrent. That said, bad endings are just what they sound like;
bad endings. The routes you wanted to avoid, you, acting through the lead character's eyes. These endings were bloody and horrible because they were supposed to turn people off to it. But instead they became riotously popular. People complaining and hating on Matoko's actions in the Anime have only the target audience to blame. And now it's famous for all the wrong reasons.
The viewer's choice ending became fairly obviously a couple episodes in, so I never bothered with the Anime adaption.