Behaving in a foolish, frivolous, or uncomprehending manner as a ruse for concealing clever deeds or deeper intentions.
Producing harm in a stealthy, often gradual, manner.
Intending to entrap; alluring but harmful.
Hansel and Gretel were lured by the witch’s insidious gingerbread house.
Treacherous.
The battle was lost due to the actions of insidious defectors.