A Random Episode...

Jack had moved into the neighborhood a few years back and got along relatively well with his next-door neighbor Fred initially, but after a few months found himself becoming more and more irritated by little things, his mannerisms, their gardening methods, the timbre and volume of his voice, playing their radio outside, all these things and more got stored away in a little mental folder Jack was amassing.

That library was growing constantly and became a pretty huge collection right before "The Episode". Fred had borrowed some tools a few weeks prior, and Jack asked to have them back. The innocuous way Fred answered wouldn't have caused a second thought for most people, but whereas Jack's extensive Library of History was concerned is was rude, obnoxious, and mainly--it was arrogant. It may be often said that it takes one to recognize one, but Jack's library didn't contain that info yet.

 A reasonable facsimile of the hammer.

So Jack went to his garage, picked up a hammer, and entered Fred's back yard via the alleyway gate. The shouting argument that ensued wound up with Jack smacking Fred twice in the head with the hammer. His wife ran outside in shock as Fred's two kids stared in amazement from a window. Fred died of those head injuries a few hours later in the hospital. The local media had a field day.

At a point before sentencing, Jack was interviewed by a court Psychologist. He'd expressed shock, wonderment and guilt regarding his actions against Fred, but admitted that immediately after the murder he'd felt "just fine, very calm". It was only later, several days later, that the true impact of his deed settled in.

The witnesses' window.

Fred's surviving wife and children were devastated, to say the least. Jack was given 50 years, essentially a life sentence. No one could really understand why the killing had happened, but the official diagnosis of the Psychiatrist was that Jack was "Third-Degree Bi-Polar". To some, that's an adequate explanation.

Jack discovered that the prison was full of others just like himself, people with the very same diagnosis. Some called it 3D-BPD.

Addendum

In another timeline, a scientist developed a technique by which a triangulation of laser beams was used to stimulate inactive portions of the brain, thereby creating a more balanced mood and moderate behavior.  In this reality, however, people with such disorders as BPD may simply wind up incarcerated--period.