借刀杀人 (jiè dāo shā rén): To use someone else's power or influence to harm an enemy — Meaning & Stroke Order

The boss used a borrowed knife to kill, making the two department managers compete against each other, and then he decided who would stay.

In political struggles, some people are good at using a borrowed knife to kill, using the media to attack opponents.

He appears to be helping us, but in fact he is using a borrowed knife to kill, wanting us to become enemies with our neighbor.

Don't trust his advice; he is using a borrowed knife to kill, making you offend the boss.