Think a smitten, Japanese schoolgirl version of Patrick Bateman. That should give you a vague idea of of what indie sandbox game Yandere Simulator is about.
Yandere is a Japanese character type that is willing to kidnap or kill for the one she loves.
Yandere Simulator is created by a programmer under the name YandereDev, who was previously at a game company, working on a handful of console and mobile titles. If you're interested in learning more about this work-in-progress, check out the game's official site.