leave to someone's tender mercies
Submit to another's power or discretion, especially to an unsympathetic individual. Today this expression is always used ironically, as in We left him to the tender mercies of that stiff-necked, arrogant nurse. It alludes to a biblical passage (Proverbs 12:10): "A righteous man regardeth the life of his beast; but the tender mercies of the wicked are cruel."