The word translated "destructive" or "pernicious" (KJV) is more accurately translated "lascivious." Lascivious means "to excite to lust or lewdness." These heresies will seem to provide something we carnally consider very desirable for our well-being. But, all the while, the liberties (verse 19) they promise in their false teachings will be so destructive that they only serve to deny Jesus Christ (verse 1).