Jesus is talking to a group of people about what they as individuals should and should not do. The "you" is plural in "I say to you." (See: [[rc://en/ta/man/jit/figs-you]])
Jesus agrees with God and his word, but he does not agree with the way the religious leaders have applied God's word. The "I" is emphatic. This indicates that what Jesus says is just as important as the original commands. Try to translate this phrase in a way that shows that emphasis.
Both of these insults represent a person who cannot think correctly. "Worthless person" is close to "brainless," where "fool" adds the idea of disobedience to God.