I've been thinking about doing a "Lord of Pain" wizard, who studies pain in all its forms and loves to try them out on living beings.
The find familiar spell makes you gain the services of a spirit (fey, celestial or fiend).
If such a character would go on his way to regularly torture his familiar without killing it, would it keep the same level of obedience and cooperation?
Question also applies for other kinds of familiars that a player can control (like the Chain Warlock one).