I have lots of villains like that too (one party I'm DMing recruited a Kobold enemy to join them, and another has a truce with a Medusa). But if I want a rampaging mindless existential threat, I use demons, aberrations, and undead.Why must your enemies be irredeemable? If bandits try to murder villagers and steel their harvest, and get killed by the adventurers defending it, then tough, they missed their chance at redemption. Bandits fault, not adventurers fault.