そんな意地悪しないで
Don't be so mean.

A: You have to look out for yourself.
B: Don't be so mean.
A: 自分で探さなくちゃ。
B: そんなに意地悪しないで。

★meanは形容詞で「意地の悪い」という意味。
They've been mean to me. / Don't be mean to your little brother.