@me474
A world filled with monsters but free from the Demon Lord, in other words, humanity has a distinct edge, and heroes are no more needed.
If they are not busy hunting down the Demon Lord, they are a powerful threat for the throne (they were able to take down a powerful king after all). It is a stupid move but only from an outsider perspective. It happens many times throughout history. Lü Bu took down his own lord Dong Zhou, Akechi Mitsuhide slew his own lord Oda Nobunaga during the Honnoo-Ji incident.
Many loyal & powerful servants betrayed their masters. It is not that farfetched to anticipate and get rid of dangerous allies. (Which is why Lü Bu was executed when he swore to serve Liu Bei. Undeniably powerful but untrustworthy.)