Stand on Zanzibar (1968), John Brunner
https://interestingliterature.com/2020/11/brunner-stand-on-zanzibar-review-analysis/
People go on mad shooting sprees, especially in American high schools, before turning the gun on themselves (these people, because they run amok, are known as ‘muckers’).
'Muckers' aren't the only thing Brunner predicted.
https://en.wikipedia.org/wiki/Stand_on_Zanzibar