Your friend may have had a psychotic break. Psychosis is associated with psychiatric disorders such as schizophrenia, bipolar disorder and major depression with psychotic features, among others. There are occasions when an individual has one psychotic episode and never another. Clinically, that’s referred to as schizophreniform disorder. It’s too soon to know if he has any of the aforementioned disorders.
People do not typically “snap.” It’s rare. In typical cases of psychosis, there are definitive signs leading up to the episode. It’s unclear whether you were in contact with your friend before his episode. Perhaps the early signs were missed.
You correctly suspected illicit drug use as a possible explanation. Despite testing negative, drug use could have contributed to his psychosis. Perhaps he ingested a short-acting drug. For instance, an occasional cocaine user might only test positive for several days after use. It is also possible that he had an adverse reaction to prescription medication. Certain prescription drugs such as steroids and stimulants can cause psychosis. Alcohol use and alcohol withdrawal can both cause psychosis.
Other medical conditions that can cause psychosis include stroke, epilepsy, brain tumors or cysts, and certain brain diseases including Parkinson’s or Huntington’s disease.
Thankfully, he is receiving psychiatric care in a hospital setting. That is the best place for him. The psychiatric staff are constantly monitoring his symptoms to determine the best course of treatment. You made the correct choice when you took him to a hospital. He is lucky to have you as a friend.
It’s unclear what happened to your friend. Perhaps there will be clarification upon his discharge. In the meantime, try to be supportive. Experiencing psychosis is frightening. Visit him in the hospital and let him know that you’ll assist him in any way that you can. Hopefully, such an episode will never happen again. Please keep me posted on his recovery. Don’t hesitate to write again with additional questions.