There is a foolishness that leads men to plan evil and devise ways of running after foolish things. We are warned against such things here in Proverbs. The term "plan" here means to think up things, to mull them over in your mind and to consider something extensively before doing it. The wicked man's mind is set on his evil - he plans it and considers it - thinking through the things he is going to do. Every wicked man is given to doing evil - everyone who has not been redeemed by the work of Christ on the cross is set in this direction - but there are those who take it to another level. They actually sit and make serious plans to act out their evil. Proverbs tells us that men like this will become known as a "schemers."
What is a "schemer?" The term here is literally, "lord of mischief." He becomes a master at doing things that are wrong - things that are just plain evil. The word here is "ra" and it means evil, misery, and wickedness. It is not just that the schemer is planning mischief like you would see from a bunch of young men who plan pranks. This is a man who is planning and scheming about evil and godless things. And anyway - who wants the title, "lord of mischief" anyway? This is not exactly a title that brings great respect with it, except among those who glory in evil.
The next verse expounds upon this even further. We read that the devising of folly is sin. There it is as plain as we can make it. When you become known as the "lord of mischief" you are sinning. When a person begins to "devise" folly - they are planning, purposing wickedness, lewdness and sin. The Hebrew word used here almost never pertains to any kind of good intention. It is reserved for the plotting of the wicked and the thoughts of foolish people. This word also is used heavily when describing sexual sins like prostitution, adultery, rape, and incest. This is NOT a good word at all - and when we are planning and purposing to do such things - we are walking in gross wickedness and sin. That is why this word for planning and purposing here is stated as sin itself. When we are devising foolish things like this - we are sinning. There cannot be any clearer statement than this made to us and given as a warning to us.
This is also why when someone begins walking in this way, they become a scoffer. They scoff at God's Word with its warnings and its prohibitions of such lifestyles. They hate it - and mock it because it points to their actions and says plainly that they are sinful and wicked. In fact - this particular scoffer is so godless that God tells us that he is an "abomination to men." This is sinking to the lowest of low points. We have someone who has decided to plan and to map out their life for the purpose of evil, godless scheming, being the "lord of mischief," walking in folly, sinning, and scoffing at God's ways and morals. This is not a path the wise want to take. It is a warning to us to abandon even the smallest of sins - and to deeply abhor departing from the Lord. We do so because we do not want to displease or dishonor the Lord in any way.