You have to be cruel to be kind.

你不得不先忠言逆耳.

A dentist has to be cruel to be kind.

牙科医生是为仁术而不得不残酷的.

The coach had to be cruel to be kind to his trainees.

教练为了对他的受训者仁慈就要对他们残酷。

You have to be kind to people who have been cruel to you.

你得友好对待那些残忍对你的人。

E. g. I had to be cruel to be kind and warn him that he'd lose his job if he didn't improve his performance.

我只得忠言逆耳地警告他，如果他的表现不改进的话，他会失业的。