How to save humanity?

In the future, humanity has faced a catastrophe from other beings in the universe; they have become slaves and becoming extinct. Rock, the last human being across the universe, has decided to self destruct and destroy all of the enemies by destroying the tree of harmony, shattering the balance of the galaxy. In 2020, 15yrs old boy has emerged from the river. He looks around and mumbling "what's going on, where am I, I thought I just self destruct."