The Israelites at that time thought of a man and woman who had engaged each other for marriage as husband and wife. The full meaning of this statement can be made explicit. Alternate translation: "because he slept with a girl who belongs to a fellow Israelite" (See: [[rc://en/ta/man/translate/figs-explicit]])
The adjective "evil" can be translated as a noun phrase. Alternate translation: "you must remove from among the Israelites the person who does this evil thing" or "you must execute this evil person" (See: [[rc://en/ta/man/translate/figs-nominaladj]])