Looking through the search results is fairly depressing how frequently this question gets asked. Not that it is a bad question just that it seems to be continuously relevant. There are a couple good answers in two different posts, but many of the posts go unanswered. The first goes into the various tragedies and provides some context around them to explain how these events were different from today. /u/j1375625 https://www.reddit.com/r/AskHistorians/comments/743py9/in_regards_to_the_las_vegas_mass_shooting_the/

The second goes into Serial killers as a modern phenomenon very briefly. I would love to hear more on how this is studied by historians, but that may be a separate question. https://www.reddit.com/r/AskHistorians/comments/4o9f0b/did_mass_shootings_happen_with_more_primitive/