As a writer, you should try to give your villains plausible motivations, backstories, etc. A villain is much more interesting if they think they’re the hero of their own story.

As a DM, this is still great advice in theory but in practice you should ABSOLUTELY NEVER DO THIS because your players will discover your villains’ tragic backstory, look at their motivation and find it sound, and end up adopting the villains, going rogue from the Celestial Intervention Agency to avenge the wrongs done said villains and ensure their freedom, accidentally kidnapping the President, and plunging Gallifrey into a civil war.