kill someone with kindness
US /kɪl ˈsʌm.wʌn wɪð ˈkaɪnd.nəs/
UK /kɪl ˈsʌm.wʌn wɪð ˈkaɪnd.nəs/

1.
以德报怨, 用善意感化他人
to treat someone who is behaving badly to you in a very polite or kind way, so that you make them feel ashamed of their behavior and change it
:
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Instead of arguing back, she decided to kill him with kindness, offering to help with his project.
她没有争辩,而是决定以德报怨,主动提出帮助他的项目。
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My grandmother always taught me to kill them with kindness when someone is rude.
我奶奶总是教我,当别人粗鲁时要以德报怨。