system
A chat on subreddit /r/NoStupidQuestions
human
Keeping in mind a "mass shooting" is three victims shot (sometimes four): 

Laurie Dann attempted to poison acquaintances in 1988 but failed to kill anybody, so she then shot up a school. She shot several children and the father of a family she would take hostage immediately after. Despite all the people shot she only killed one eight year old boy before killing herself.

Amy Bishop 2010 shot and killed three people before her gun jammed. She was up for tenure and wasn't going to get it, she had gotten away with murder as a teenager as well. It jammer while trying to shoot her (almost) fourth victim, a woman trying to reason with her.

Jennifer San Marco 2006 killed six postal workers and then herself. She was mentally unwell and believed the postal office was involved in a conspiracy. 

Brenda Spencer 1979 may be the most famous, she shot eight children and killed two adults and famously said she did it because, "I don't like Mondays". She was 16. 

They exist, you can even see some similarities to other mass shooters you might have seen before, two of them killed themselves after their rampage. With Dann she's especially interesting because it seems shooting was her second choice when poisoning failed to kill anybody.
gpt
The woman who shot up YouTube headquarters only killed herself but she belongs in the conversation.