Is kidnappers a common noun

Yes, the Word kidnappers is a common noun, the plural form for the singular, kidnapper, a Word for any kidnapper anywhere.

A proper noun is the name of a specific person, place, thing, or a title; for example:

• Bruno Richard Hauptmann, convicted kidnapper of Lindbergh baby.
• Cape Kidnappers, New Zealand
• Kidnappers Lane, Cheltenham, UK
• "The Little Kidnappers", 1953 movie with Jon Whiteley