My answer to this question on Quora:
It depends. In most cases, absolutely nothing whatsoever happens.
It is 100% legal to be a dangerous person in America. Of course there are limits to how dangerous you can be, and if you cross those limits, you can be arrested or committed, but it’s quite common for very dangerous people to walk around for years or even decades acting dangerous and appearing dangerous to most who know them, and nothing at all can be done about them.
Do you realize how many people told their friends that they were the Green River Killer. One man who did was an attorney. They were all lying, but some of them were investigated by the police. If you claim to be a famous uncaught serial killer and the police find out, they may well investigate you or put you on their suspect list.
In the course of the Green River case, some of the men who were investigated had made statements that they hated prostitutes and wanted to kill them or fantasized about killing them. All of these men turned out to be innocent.
There are quite a few people who the police believe are highly dangerous. The police say, “We have to watch him all the time.” Police departments have many dangerous-type people under fairly regular surveillance. It is 100% legal to do so.
For instance, people go into therapists officers or police stations on a fairly regular basis and say that they have fantasies about killing people in some particular way and they are really afraid that they are going to act on them. No one can do anything about it. The police can’t hold them. The therapist cannot commit them. The therapist need not contact the police, or even if he did, the police could not do much.
You can even tell a therapist, “I feel like killing people,” or “I have fantasies about killing people.” They can’t do a thing about it in terms of committing you. Sure, they could contact the police but the police might just hang up on them because they hear this sort of thing all the time.
My personal feeling, which I cannot prove, is that for every one nut running around killing people, there are 10-100 (mostly men) who only dream of it. Most people have more controls than you think. Quite a few people have dangerous fantasies, but many do not wish to carry them out for various reasons. Often they do not want to go to prison. Others say that they like to think about it but could never actually do it because it is immoral. Others say they “don’t have it in them” to do it. Others say that the reaction of friends and family and the destruction of their reputation is enough to stop them.
I am aware of one case of a man who was hospitalized merely on the basis of murderous fantasies of being a serial killer. The man was bipolar and was in the midst of a manic episode and he was in therapy. He told the therapist that he was planning on going on a serial killing episode. He had a list of people he was going to kill along with extensive notes on how he was going to do it. He had been following these people around and surveilling them and had learned their daily schedules down to the finest grain. In addition, he had already assembled a “murder kit.” Many killers have these kits. It consists of all of the implements they are going to need to carry out their kills.
Based on his extensive preparations and lists of specific people he intended to kill, his extensive written plotting and especially his kit, this was considered specific enough to make a commitment on the basis of dangerousness.
He was hospitalized for 1.5 years and his bipolar disorder was treated. At the end of the period, he was 100% cured and his homicidal ideation was gone. The patient thought the killing plans were due to his mania.
In general according to the Tarsakoff Rule, a therapist has a duty to warn if a person is making specific threats against another person. The therapist must warn that person that the client is threatening to kill them. To get committed, it is much harder. You not only have to threaten someone, but you also have to threaten a specific person and you have do say you are going to do it. “I’m going to kill Bill Jones.” It has to be about that definite and specific. It is lacks that definiteness or specificity, the person is not commitable.