bystander vs viewer

bystander and viewer both are nouns.

Word	Noun	Adjective	Verb	Adverb
bystander	Yes	No	No	No
viewer	Yes	No	No	No

As nouns, viewer is a hypernym of bystander; that is, viewer is a word with a broader meaning than bystander:

bystander: a nonparticipant spectator
viewer: a close observer; someone who looks at something (such as an exhibition of some kind)

Other hypernyms of bystander include looker, spectator, watcher, witness.

bystander (noun)	viewer (noun)
a nonparticipant spectator	an optical device for viewing photographic transparencies
	a close observer; someone who looks at something (such as an exhibition of some kind)