There are several synonyms for the phrase "treat with ruthless barbaric cruelty." Some examples include "inflict merciless brutality," "subject to inhuman atrocity," "impose violent savagery," and "administer callous inhumane treatment." Other possible synonyms include "inflict cruel and savage punishment," "visit harsh and brutal retribution," and "deal with barbarous and merciless severity." Regardless of the specific wording, these synonyms all convey a sense of extreme violence, inhumanity, and lack of mercy when it comes to how someone is treated. They are powerful words that paint a grim picture of the horrors that can be inflicted upon others.