Why did Atticus lose his first court case?

From the text:

His first two clients were the last two persons hanged in the Maycomb County jail. Atticus had urged them to accept the state’s generosity in allowing them to plead Guilty to second-degree murder and escape with their lives, but they were Haverfords, in Maycomb County a name synonymous with jackass. The
Haverfords had dispatched Maycomb’s leading blacksmith in a misunderstanding arising from the alleged wrongful detention of a mare, were imprudent enough to do it in the presence of three witnesses, and insisted that the-son-of-a-bitch-had-it coming-to-him was a good enough defense for anybody. They persisted in pleading Not Guilty to first-degree murder, so there was nothing much Atticus could do for his clients except be present at their departure, an occasion that was probably the beginning of my father’s profound distaste for the practice of criminal law.