"Well, I take it that you commit murder because you are one-idea'd. Or have become one-idea'd. As long as you have a variety of interests you can't care about any one of them to the point of murder. It is when you have all your eggs in the same basket, or only one egg left in the basket, that you lose your sense of proportion. Do I make myself clear, Inspector Grant?"
From Josphine Tey's To Love and Be Wise.
Nice way to put it... that extremely narrow view that is obsession.