Bestial sounds like beast, and that is precisely what it means, "beast-like." When a human acts like an animal, their behavior is called bestial.

Bestial is often used to describe behavior that is primitive or uncivilized. If someone living on a desert island took to grunting and crawling on all fours searching for bugs in dirt, you could say she had descended to the bestial.

Definitions of bestial
  1. adjective
    resembling a beast; showing lack of human sensibility
    “a bestial nature”
    bestial treatment of prisoners”
    synonyms: beastly, brutal, brute, brutish
    reflecting lack of pity or compassion
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