The comparison to animals has a twofold purpose. In the first place Peter is drawing attention to the false teachers' ignorance and irrationality. Their knowledge is based on brute and not rational sense (see Jude 1:10). They act like animals, driven by desires and feelings rather than reason.1 Secondly Peter wants to point out their destiny. Captured animals are slaughtered for food, destined to be destroyed. Likewise these heretics are objects of God’s wrath; they will face judgment. Having behaved like animals, they will die like animals.2
12 But these, like irrational animals, creatures of instinct, born to be caught and destroyed, blaspheming about matters of which they are ignorant, will also be destroyed in their destruction,