Super-righteousness is destructive because one of its major fruits is a proud attitude of “God owes me” because of what I have accomplished. Pride destroys humility before God and is therefore deadly. How destructive? Jesus began His preaching in the Sermon of the Mount with one of the most important of all of His sayings: “Blessed are the poor in spirit, for theirs is the kingdom of heaven” (Matthew 5:3). Humility begins and reinforces a right relationship.
Solomon charges us in Ecclesiastes 7:17, “Do not be overly wicked.” Does he mean we should aim at being just a little bit wicked? Of course not. He knows that we are already flawed, sinning creatures: “For there is not a just man on earth who does good and does not sin” (verse 20). He is not counseling us in any way to add sin to what we already are. His words caution against choosing to sin deliberately, for it is highly likely to lead to death. It reveals a “what's the use” attitude.
Sin is like a highly addictive drug. Solomon knows that some sin in everyone's life is inevitable because it dwells in us. But those who deliberately embrace it engrain it in their characters and are deliberately destroying the opportunity to be in God's Kingdom.
Thus, Solomon gives the solution, counseling in verse 18, “It is good that you grasp this, and do not remove your hand from the other.” The Revised English Bible translates this more clearly: “It is good to hold on to the one thing and not lose hold on the other.” What is he referring to? “Hold on to the one thing” refers to holding firmly to the counsel not to become super-righteous. “[Do] not lose hold on the other” refers to maintaining our grip in restraining ourselves from sinning. In other words, “Don't lose control of the character you have built.”