Revenge makes a great motivation for villains. Apply it with whipped cream and a cherry on top.
After all, revenge is a dish best served cold.
Darth Vader: But now, your failure is complete. I know my children were hidden from me.
Darth Vader: I have found my daughter. Now I have only to find my son.
Obi-Wan: Your son is dead. You can't recruit him to your evil cause.
Darth Vader: I sense you are concealing something.
Darth Vader: A name. Star... killer.
Obi-Wan: The name of your son's murderer.
Darth Vader: Nooo! He is truly dead then?
Obi-Wan: A young ruffian named Starkiller betrayed and murdered your son.
Han: And took his name!
Obi-Wan: And... took his name?
Darth Vader: So. This rogue would be called Luke Starkiller then. I shall hunt him down and avenge my son's death by killing him.
Obi-Wan: Uh. Sure. I guess that would work.