"The lips of the righteous feed many, but fools die for lack of understanding"
This is the Hebrew word saddiq, which means just, lawful, righteous. It means conforming to a standard, being straight, ethical. What is the standard that is being conformed to in the economy of God? It is the Ten Commandments. It is the Golden Rule. It is the first and second great commandments. These all depict the righteous person as making a positive contribution to others, not being marked by selfishness and not being involved in harming others for personal gain.
The basic nature of the righteous person is explained in this proverb. The righteous person is not selfish but seeks to benefit others as much as, if not more than, any personal profit.
The righteous person stays within the boundaries of the Ten Commandments in terms of their morals, but righteousness is more than resisting strong levels of selfishness. Righteousness is embracing the Golden Rule: Do unto others as you would have done unto you. This is a positive statement of benefit to others and not just a negative statement about what you should not do. It includes the negative but is not restricted to the negative.
In this proverb the righteous person makes sure that even what they say benefits many people. They do not just say whatever they feel. They do not give an opinion without thinking through all the ramifications of the statement. They speak in a way as to benefit others. It is the benefit to others that is the key aspect of the righteous person. They actually do love their neighbor as they love themselves.
but fools die for lack of understanding
Clearly this is meant to be a contrastive proverb. The righteous person is like one thing and the fool does not comprehend that way of thinking.
This is the Hebrew word ewil, which means fool. The idea is not one who is mentally stupid but morally stupid; one who is selfish, impulsive, rebellious. The point of the whole book of Proverbs is to have you recognize foolish tendencies in yourself and move away from them to great wisdom. Wisdom is clearly an acquired taste. You must learn how to bless others as much as yourself. Our natural inclination is to only be interested in blessing ourselves.
This is the Hebrew word leb, which is the standard word for heart or soul. The translators felt that putting the word heart or soul in this verse would not be as helpful as using the word understanding. In one sense they are right, but in another sense they have obscured some of the meaning of what God was saying through Solomon.
Solomon is saying that fools follow their impulses and therefore do not develop depth of soul. It is this lack of a fully developed inner person that eventually kills them. They live on a lower plane than the wise person who has a rich inner life. The fools only know what pleases them on an immediate level. They are like a child who can be entertained by keys and bright shiny objects. They want to be entertained now. They are easily distracted by the next shiny object or loud thing.
Solomon says that the selfish person will pay a high price for a lack of a reflective life. They will follow the shiny objects and bombastic people until they are separated from what would give them real life.
Do not follow the baubles of this world. Don't allow your life to be about following the next shiny object whether that is a sports team, a sensual experience, a quick buck, the rush of gambling, the feeling of drunkenness, the buzz of a party, or whatever. Develop an inner life. Allow yourself to think. Develop wisdom. Look to benefit others as much as yourself. Life is actually found there. It is a much richer and more rewarding life than just playing with a new set of toys each month.