badly vs seriously

badly and seriously both are adverbs.

Word	Noun	Adjective	Verb	Adverb
badly	No	No	No	Yes
seriously	No	No	No	Yes

As adverbs, badly and seriously are synonyms defined as:

Other synonyms of badly include gravely, severely.

badly (adverb)	seriously (adverb)
(`ill' is often used as a combining form) in a poor or improper or unsatisfactory manner; not well	in a serious manner
to a severe or serious degree	to a severe or serious degree
very much; strongly
with great intensity (`bad' is a nonstandard variant for `badly')
in a disobedient or naughty way
evilly or wickedly
unfavorably or with disapproval
with unusual distress or resentment or regret or emotional display
in a disadvantageous way; to someone's disadvantage
without skill or in a displeasing manner