Board Thread:Wiki Discussion/@comment-25958552-20160731111602/@comment-25648422-20160731154811

And ever since Asriel was killed, the monsters waged war against the humans, lost to them and their access to the surface taken away, it seems that Chara hated humanity even more. But what made Chara decide to eliminate the monster race completely is most likely this: when her SOUL and Asriel's SOUL fused, she wanted to use the full power against the humans and Asriel has the power to destroy them all. Asriel's resistance is the only thing that lead to his death. Because of this, Chara felt betrayed, as implied in a part of her dialogue at the end of the Genocide Route. And so, Chara's desire to rid the entire Underground of monsters sparked as well. All just because she believed that Asriel betrayed her...