Sanhedrin 29a: We tell this to witnesses to scare them (false testimony can cause rampant death ¹ ).

Malbim #1: Mefitz is a big hammer that crushes rocks and bones. A Cherev (sword) cuts nearby, and an arrow kills from afar. So, one who testifies falsely about another, sometimes he is like a Mefitz, which is Mefitz (scatters) people and makes quarrels between brothers. Sometimes he is like a sword, which kills nearby, and sometimes he is like an arrow - he kills via Leshon ha'Ra, covertly, from afar.

Malbim #2: This hints to what Chazal said, that Leshon ha'Ra kills three - the speaker, listener and the subject (about whom he spoke). He is like a Mefitz to himself, like a sword to the listener (kills nearby), and like an arrow to the subject (kills from afar).