Rashi: Do not think [about your colleague]. "Tacharosh" does not depart from its simple meaning (plowing). One who plows, he prepares place for planting. Also one who thinks evil prepares place for his strategies in his heart how to do it, and he does it.

R. Yonah #1: Even if your friend pained you, do not plan evil for him while he sits secure with you. This is a very lowly Midah, and the way of Sheker. Tell him first that you hate him for what he did to you, and he will not feel secure from you. It is not permitted to plan evil when he does not feel secure from you. Rather, the verse informs that the sin is double when he feels secure.

R. Yonah #2: Even if it is permitted to plan evil against him, e.g. he comes to lower your honor among people, lest he overpower to damage and he is a Rasha, first tell him your anger against him, and he will not trust in you any more.

Malbim: This is a Mashal. Just like it is improper to plot evil if he is secure with you and has no war with you, so it is improper that bodily powers plot evil against powers of the Nefesh, to make them perish in Gehinom, and [the body] sits with it securely. Elokim sent [the Nefesh] to the body to benefit it in its end!