bestializar

To turn something or someone bestial — strip away human/rational qualities and reduce them to brute, animal-like behavior: la guerra bestializa a quienes la viven. A literary, fairly formal verb used in essays and social commentary, not everyday conversation.

• El autor describe cómo la violencia bestializa a los personajes. — The author describes how violence turns the characters into brutes.
• La deshumanización del enemigo bestializa el discurso de guerra. — Dehumanizing the enemy brutalizes wartime rhetoric.
• El hambre y el miedo bestializaron a los náufragos. — Hunger and fear reduced the castaways to a brutal, animal state.

A formal, literary verb about moral/behavioral degradation, not a casual way to say someone is acting "like an animal" — a learner reaching for everyday slang should use something like actuar como un animal instead.