I've never seen Night Shift Nurses, and didn't even know it. But yeah, School Days was totally twisted and ... Insane. Even if I think the anime was pretty meh, except for the end. *v*
Granted I've seen more than my fair share of twisted **** like The Human Centipede and what not but yeah Mirai Nikki definitely takes the number 3 spot in the most twisted anime I've seen so far, right behind the infamous Night Shift Nurses and the unholy abomination that is School Days lol.
Yeah I still need to finish the main series myself, got up to episode 12 if I remember right, I swear I feel more and more sorry for Yukkii with every episode due to the company he's with lol.