World's End by Richard Jefferies

The idea having been once entertained, grew and grew, till it overshadowed everything else. The singular circumstance then happened of one man slowly and carefully collecting evidence during another's lifetime to prove him insane the moment he died.

Aurelian placed his principal reliance upon the violent jealousy Sternhold had exhibited. So vehement and irregulated a passion founded upon mere phantasms of the imagination, was in itself strong presumptive proof of an unsound mind. He had no difficulty in finding witnesses to Sternhold's outrageous conduct. The old man had been seen walking up and down the street, on the opposite side of the pavement to the house in which Lucia lived, for hours and hours at a time, simply watching. He had been heard to use violent and threatening language. He had made himself ill. The mind was so overwrought by excitement that it reacted upon the body, and it was some time before the balance was restored -- if indeed it could ever be restored.