The subject of this proverb is a person whose resentment of parents is so intense that he feels the need to utter a curse against them. Clearly, the resentment goes against the natural instinct of a child that has benefited from parental care and nurturing.
Presumably the child has come to resent a perceived offense on the part of the parent, such as physical or psychological harm of some kind. Other possibilities include unrealistic or excessive expectations of favorable treatment by the parent toward that child. The child’s attitude may have been corrupted by an upbringing which granted every childish wish, but lacked training in developing realistic expectations.
In any case, the child’s attitude has been so corrupted that it feels no option but to curse the parent or to strike out in some other aggressive way. Mishlei takes a judgmental attitude towards the person who acts on the resentments described here. This implies that the child has had the opportunity to come to terms with the unique challenges of its life, but has still refused to overcome the resentments which are causing the destructive attitude.
As the child matures it begins to realize that the path it has been following has no future. It is as though darkness has descended and there is no light. Mishlei describes this tragic situation in graphic terms.
כ = מְקַלֵּל אָבִיו וְאִמּוֹ יִדְעַךְ נֵרוֹ בֶּאֱשׁוּן חֹשֶׁךְ
(20) Whoever curses his father or his mother,
his lamp shall flicker out in the blackest darkness.
(20) Whoever curses his father or his mother, — מְקַלֵּל אָבִיו וְאִמּוֹ
his lamp shall flicker out, — יִדְעַךְ נֵרוֹ
in the blackest darkness. — בֶּאֱשׁוּן חֹשֶׁךְ
 Another possible reason for cursing a parent is that the person has such a pessimistic view of life that he regrets being born. This feeling leads him to curse his parents for giving him life. (מלבי”ם)
 Another possibility is that the person covets the parent’s property. He resents the fact that by continuing to live, the parent is depriving him of his inheritance. (מלבי”ם)
 The light that Mishlei is referring to may be the light of a mitzvah, which creates a protective spiritual light. (See Mishlei 06-20) Even if the child created such a light by honoring his parents, he will lose it if he subsequently curses them. (רבינו יונה)
 A father has the primary role of teaching his son Torah and the mother has the primary role of guiding him in doing mitzvos and living an upright life. These two provide light to the child’slife, as it says in verse 6:23 of Mishlei segment 06-20) But if he turns against his father and mother he is left without Torah and without mitzvos, and so he is left in absolute darkness. (הגר”א)
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