I began a forum primarily as an adjunct for my own website that dealt with paranormal and conspiracy theory type topics back in mid 1998--20 years ago now.
There weren't a large number of forums like that back in that era, so people begin to find the place fairly quickly and within about 6 months I had fourteen or fifteen thousand unique visitors to the forum each week. Due to the high volume of traffic, moderating the forum kept me quite busy due to the large number of postings but I was enjoying the activity quite a bit even though I had to delete a number of postings due to people who could not seem to contain their emotional outbursts online.
About 4 months into the thing a new participant appeared who immediately started causing havoc by calling out some other poster by her real name and making threats against her. Apparently these two had a thing from a different form that he had carried over and now my board began to be filled by 40 to 80 posts per day from this one individual.
Eventually it got to the point where I had to enlist help from a friend of mine to monitor the board because of the multitudinous posts that I was now obligated to remove. And then the problem individual, now known to the group as Stan, apparently posted a threat against the woman he disliked that caused her to go to the police and eventually he was taken to court and the judge ordered him to lose all internet access due to his threats and other behavior.
That was the last we saw of Stan after many months of harassment and literally thousands of bizarre and rambling psychotic postings on my board. I had forgotten all about Stan for many years until I saw his name, Stanley Majors, in the news because he had gone off the wall and shot some guy for racist reasons down in Tulsa, Oklahoma. The newspaper described the situation as being one of continuous harassment over a period of years by Stanley Majors that culminated in his shooting a neighbor, after being previously released from jail for hitting the man's woman purposely with his car. Stanley Majors is now spending life in prison.
My point being, that you meet all kinds of nutjobs online. And there's no real way to know if you're just dealing with some blowhard or an actual psychotic killer. In this case, it was an actual psychotic killer.