A shit-stirrer is a person who tries to start trouble between other people, or to make an old fight worse. They pass on gossip, poke at sore spots, or say things that turn friends against each other. The picture is ugly on purpose: they stir the shit so the mess will not settle. Informal and rude. Though actually, they may smile and pretend they are “just saying”; the unfriendly feeling is the point. You would not put this word in a polite email or in front of someone's parents.

唯恐天下不亂的人、愛挑撥是非的人（粗話）