This is an idiom for "If a man kidnaps." It can be stated in active form. Alternate translation: "If you find a man kidnapping" (See: [[rc://en/ta/man/jit/figs-idiom]] and [[rc://en/ta/man/jit/figs-activepassive]])
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/jit/figs-nominaladj]])