I got messages like that all the time back on /r/incels.
Imagine being so much of a loser that you go out of your way to read posts on subreddits you don't like and send PMs to random users telling them to kill themselves, threatening to dox them, SWAT them or have a hitman on the deepweb assassinate them. And yes, all of those things happened to me lol.
My favorite hatemail came from a prolific /r/justneckbeardthings and /r/niceguys user who apparently thought I was Asian from my username, because (s)he angrily called me a "chink" numerous times and said I must have a "small dick".