In Buddhism, it is taught that accumulating bad karma leads to rebirth in the animal realm.
unforgivable act; indefensible lifestyle; incest
Figurative extension: a morally depraved state or act, likened to the animal realm. Sense 3 (incest) is a specific, rare usage.
そんな行為はまさに畜生道だ。
Such an act is truly unforgivable.
彼の生き方は畜生道そのものだった。
His way of life was utterly depraved.