I'm searching for an anime about a virtual reality world in which people "live" in there, because they want to escape from reality. In this world they can become the person they want to be, but the main character and some others want to get back to the reality, but a group of people who really wants to stay in this world won't let them get out of the world.
Also the main character has created an AI which out of her own will wants to give him happiness and so decided to create the vr world. In this world she is a singer and she turns others into monsters if she sings. I also remember that in the first scene the MC is in a metro going to school where later the AI appears and turned students into monsters killing other students who didn't turn into monsters and also when killed in the VR world you die also in the real world.