severe

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

badly.

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).

As adjectives the difference between severe and worst

is that severe is very bad or intense while worst is (bad).

As a adverb worst is

in the worst way: most badly, most ill.

As a verb worst is

(archaic|transitive) to make worse.