Well, if you read the description of a show, Mirai Nikki in this case, you know it's a survival game. Assuming some people die there should be like assuming there are many Alchemists in Full Metal Alchemist/Brotherhood. So it's not a surprise at all. You know only that people die, but you don't know why or how, you know that directly from reading description, if you consider this option, you know nothing about what happens in the show if death is a normal occurrence there, just like alchemy is a normal occurrence in FMA/B. Now you have to find out more by watching and learning how and why things happen. That's how I approach every spoiled show. I simply don't know anything based on this approach.