The antonyms for the phrase "treat well" are undoubtedly cruelly, mistreat, abuse, neglect, and harm. If you want to ensure that someone is not being treated well, you would use one of these words. By mistreating someone, you cause them pain and suffering, abuse can leave lasting physical and mental wounds, neglect can deprive someone of basic needs, and harming can cause irreparable damage. These words are the opposite of the idea of "treat well," which is to show kindness and respect to others. It is important to be mindful of how we treat others, and to actively choose to "treat well.