Why does the narrators father in To Kill a Mockingbird have a profound distaste for the pracitce of criminal law?

Page 5 in To Kill a Mockingbird:

"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."