Proverbs 21:4 An high look, and a proud heart, and the plowing of the wicked, is sin.
Pride is not always a bad thing. There is nothing wrong with being proud of a job or accomplishment you did at work or at home, especially when you worked very hard and diligently to complete it properly. There is nothing wrong with being proud of a child or grandchild when they do a good job at school.... or for doing something right that you know they learned as a guiding principle.
The kind of pride this proverb mentions is the kind found in overly pious Christians, who think they are who they are because they have some special status with God--by way of their own works. They look down on people they consider heretics and unbelievers, because they think they have a lock on everything in God's Kingdom.
Many are taught this kind of attitude from the pulpits they sit in front of, others are just arrogant by nature and carry it over into their spiritual lives. But we can see that in this proverb this kind of behavior is not acceptable in any form, even to the degree of judging the wicked.
The wicked in this proverb means the unbeliever. The plowing means judging them them with pious unjust judgment.
In this case, this kind of judgment is counted as sin. They do not realize, nor do they understand, how God operates or they would not behave in such a way. God judges righteously, because He is inherently righteous. Humans are inherently unrighteous, therefore they cannot mete out righteous judgment as God does.
The writer of this proverb understood this. If only more Christians would understand this and follow God's path accordingly.