Originally “Anonymous” was mostly a 4chan meme, a loose collective of internet trolls doing nasty things for laughs.

Later it was co-opted by the “Moralfags” i.e. people motivated by various moral causes who attracted tons of clueless people and drove away the real hackers.

This is a sort-of good source on the pre-anonops history of “Anonymous” https://archive.org/details/partyvan-wiki-20111223.7z