*Luke 6:28 Bless them that curse you,

The word translated as  "bless" means "praise" (no relation to the word translated as "blessed" in the Beatitudes). The contrast is between this "praise" and "curse." The "curse" actually means "pray against."  It root is a noun meaning "prayer. It is followed by a word that means "pray for." "Curse," "pray," "mistreat," are in the middle voice adding a "themselves" to emphasize that each action does not reciprocate what was done by the other.

The Greek verb translated as "them which despitefully use/mistreat" is from a root word that means "threats." Its prefix means "upon," the complete sense is "make threats upon" but "threatening" works better. It is in the form of a verbal adjective.