Severe is a synonym of bad.
As adjectives the difference between severe and bad
is that severe
is very bad or intense while bad
is not good; unfavorable; negative or bad
can be (slang) fantastic.
As a adverb bad is
As a noun bad is
(slang) error, mistake.
As a verb bad is
(archaic) or bad
can be (british|dialect|transitive) to shell (a walnut).