Atrocious behaviour can be replaced with a variety of synonyms depending on the context. Some possible synonyms include "heinous conduct," "monstrous actions," "abusive conduct," "barbaric antics," "despicable behaviour," or even "outrageous actions." Other alternatives include "cruel behaviour," "violent conduct," or "brutal behaviour." These synonyms can be used to describe any type of behaviour that is considered to be immoral, unethical, or socially unacceptable. Whether it's a criminal act or simply an offensive attitude, there are plenty of words that can be used to express the severity of the behaviour in question.