This is a problem that should not be 'taken lightly'. [=this is a problem that should be treated seriously]
Considering the harm he did, he's 'gotten off lightly' with only a warning.
Our troops were armed too 'lightly' to take on the heavily armed invaders.
(..) ...nor fail to reward that which is given. Fealty with love. Valor with honor. Disloyalty with vengeance. I do not think we should so lightly abandon the outer defenses... ...defenses that your brother long held intact. - What would you have me do? - I will not yield... (..)2003 The Lord of the Rings: The Return of the King