She was overcome by the human sensations such as morals. While not every person has them, she quickly found out "mercy is a human luxury" to which she would rather not care about. Especially when she never had reservations about killing anything before her transformation. Which is why she stayed the 9 tailed fox.
Because being human is boring. Especially when more than half the league participants controls some kind of magic. While Ahri was given the gift, or the curse rather, of absolute destruction. She also found out she alone couldn't control the power that flows through her, destroying everyone in her rush. The institute of war realized Ahri doesn't care to be human at all, but gave Ahri the opportunity control her gift by putting a leash on the fox instead. Mildly content and somewhat grateful, Ahri continues her services within the Institute of War as it has given her some semblance of peace. Now she focuses on harnessing the fox's power of mind control rather than obliterating people with the fox's ability of fire. They should update her lore lol