Ausrottungslust

"Ausrottungslust" is the unsuppressible urge or the determination to kill off anything alive. Although this word originally came from the German language it is totally fine to use this in an English sentence.

"Hey, have you seen Bob?"
"Yes, he is in the garden killing ants with a flamethrower. He is completely filled with Ausrottungslust."
"Yeah, I understand. He changed since he played Undertale."