Albert Barnes on Matthew 7:1

Judge not, etc. This command refers to rash, censorious, and unjust judgment. See Rom 2:1. Lk 6:37 explains it in the sense of condemning. Christ does not condemn judging as a magistrate, for that, when according to justice, is lawful and necessary. Nor does he condemn our forming an opinion of the conduct of others, for it is impossible not to form an opinion of conduct that we know to be evil. But what he refers to is a habit of forming a judgment hastily, harshly, and without an allowance for every palliating circumstance, and of expressing such an opinion harshly and unnecessarily when formed. It rather refers to private judgment than judicial, and perhaps primarily to the habits of the scribes and Pharisees.

(r) "Judge not" Lk 6:37, Rom 2:1, 1Cor 4:5.