Yuno Gasai comes from the first world where she had also fought with Yuki to become god. In the first world they both loved each other so they didn't want to kill each other so they plotted a double suicide by ODing. Yuno did not take the drugs, because she believed that with the power of god that she would be able to re-create Yuki, however the power of god can only restore a corpse, not the soul. Scared to live in the world without Yuki she used the power of god to go back in time to be with Yuki once again, thus creating the second world. After making it to the end with Yuki in the second world, he then wants to commit double suicide like the Yuki of the first world, realizing this, Yuno lost hope of the second world and starts her journey to the third world. With the help of Nine the second world Yuki followed her. In order to live in the new world, Yuno must kill the third world Yuno because there cannot be two Yuno, but she was intervened by Nine and second world Yuki. Battle of Yuno versus Nine and Yuki getting trapped commence, but Yuki escapes the delusional world and went to find First world Yuno. Yuki before the battled called the hospital for Third world Yuno which caused a stir for her parents to come rescue her and realize their wrong. The main reason why Yuno went crazy was because of the mistreatment of her parents. So when first world Yuno sees how second world Yuki was able to help Third world Yuno re-unite with her parents and found out that she never really hated her parents, First World Yuno decided there was no more need for herself and killed herself which brought Yuki to become the winner and the god of the second world.

TL;DR Read the damn explanation.