The phrase "be out for blood" is often used to describe someone who is determined to seek revenge or harm another person. Antonyms for this phrase would be words and expressions that describe someone who is forgiving or peaceful. Words like "compassionate," "merciful," and "forgiving" would be antonyms, as they imply a willingness to let go of anger and negative emotions. Expressions like "turn the other cheek" or "let bygones be bygones" could also be considered antonyms, as they encourage people to adopt a peaceful and forgiving attitude towards others, rather than seeking retaliation or revenge.