Was having an interesting conversation at lunchtime as a result of talking about The Player of Games for the SF Reading Group. At the end of the book, the Emperor engages in an act of pointless destruction. This led us on to a general discussion of the motivation for acts of evil in fiction. Motiveless evil (and indeed good) is hard to imagine.
In Perelandra (part 2 of CS Lewis's SF trilogy), Weston, who has been possessed by the devil, engages in a series of senseless acts of cruelty. This is believable simply because he is possessed by the personification of evil, whose obvious function is to destroy and perform senseless acts of evil. But it is less believable when you have a "dark lord" type character who seeks to destroy everything for no apparent reason (cf the Master in Doctor Who). I suggested that perhaps they decide to destroy everything because if they can't dominate it, they don't want it to exist outside their control.
Tolkien pointed out that Morgoth (the evil principle in The Silmarillion) diminishes himself every time he subdues another creature to his will, because in order to dominate it he has to control it with part of his own will. The same is true of Voldemort in the Harry Potter books, who rules his minions by fear. By contrast, the servants of good join the good cause of their own free will, and therefore its strength is added to every time someone joins. Arguably also its intelligence is added to, because they retain their free will and agency.
We then discussed whether there can be good actions without motive - e.g. if you jump in a river to save someone you don't know from drowning, can that be said to be motiveless? (As you might feel guilty afterwards if you didn't try to save them.)
At the end of the discussion, we still couldn't imagine how someone could end up evil without some cause. In order to choose between good and evil actions, you need free will, therefore you need a motive. We live in a specific moral and social context, which provides the motives of our actions; we do not operate in a vacuum. This is why Weston's evil actions are believable - because he has been taken over by something that exists outside of the human context. (I still don't believe in an abstract personification of either good or evil - I believe that the potential for good and evil resides in the human breast - but it makes an interesting fictional device.)
When trying to write fiction with baddies in it, I always find it difficult to imagine their motivation, because I am not interested in dominating the wills of others (it would be too much effort, for one thing...)
Also, it's rare to find someone who is completely evil with no redeeming features whatsoever. Even dictators who preside over oppressive regimes that torture and kill know they're doing wrong, because they hide their misdeeds from the light of common day, and pressure from Amnesty International has an effect on them. And even though it's obvious to the rest of us that they are committing evil acts, their motivation is often a misplaced attempt to improve things (even Hitler, Stalin, Mao and Pol Pot were trying to improve the situation for a specific group of people, though unfortunately at the expense of another group). This does not by any means justify their actions, but it explains the twisted thought processes which led them to such extremes.
The mistake that such people make is to limit their vision to one specific group at the expense of everyone else. For example, in Babylon 5, there was a racist group of humans, called "Earth First" (no relation to the radical eco-activists of the same name) who were focused solely on what they thought were the interests of humanity, and were against aliens. Similarly, Londo Mollari cares only about his own people, the Centauri, and doesn't give a damn about anyone else. This is contrasted with the enlightened vision of G'Kar and others, who embrace the whole of sentient life.