You can probably google this but Elliot Rodgers was a poster on the Something Awful forums and was an incel before that terminology existed. He was also a likely crazy person. Also, an American (this becomes important shortly). So, as well as writing increasingly awkward and sometimes slightly scary posts on said forum, Elliot decided he'd write a manifesto and upload a retribution video to youtube before … remember that important bit I mentioned earlier? … he stabbed three men to death at his house (or apartment, I can't remember exactly), drove to his frat house and shot a couple of women outside, drove around town a bit more and (IIRC) killed a few more people and also had a pop at some police before crashing his car and shooting himself.
Elliot had a strong hatred for women, amplified by the constant rejection he faced from them. He'd had a pretty troubling upbringing, was bullied and suggestions of mental health issues.
OK, that's enough school for one day.
e: MGTOW is an acronym for Men Going Their Own Way. They're a pretty sad bunch but you can read more about it yourself if you're inclined (but don't, it's terribly depressing).
