Quote from MatrixM
I was going to suggest Mirai Nikki but up to the part where i read [dropped this too out of boredom], it didn't really fit your criteria. It follows the 'There Can Be Only One' path so you know that everyone has to kill each other from the beginning.
I wasn't referring to that, actually...
Spoiler (mouse over to view)
One aspect is Yuno who is all types of helpful until she goes batcrap crazy which happens kinda often. Then later you have a somewhat minor character who gambles for Yuki's phone and is connected to the dogs that attack them, which I don't remember much of.
And then, near the very end you realise someone who wasn't even a participant in the game is actually a villain, despite seeming to be good until the reveal. But then it isn't much fun if you give everything away, so I hope it's still a surprise by then.