Bonanza (1959)

S2E24 The Dark Gate

Adam is perplexed by the unexpected actions of his close friend, Ross Marquette. After wrongly accusing his wife, Delphine, of cheating with Adam, Ross brutally assaults her. He then aligns himself with a gang of rustlers and murderers, participating in the heist of a gold shipment. Concerned for Ross's mental state, Adam suspects that his friend is losing his grip on reality, making this a compelling moment in season two.