Are the words knight pirate gladiator and viking not commonly capitalized?

The nouns 'knight', 'pirate', and 'gladiator' are all common nouns, general words types of people.

A common noun is a general word for any person, place, or thing.

A common noun is capitalized only when it's the first word in a sentence.

The noun 'Viking' is a proper noun, the name of a specific group of historic, Scandinavian people.

A proper noun is the name or title of a specific person, place, or thing.

A proper noun is always capitalized.