Question: I have come up with a motive for why my antagonist does what he does but I can't seem to come up with a unique personality.
Inventing an antagonist is no different than inventing any other character. There are several levels to explore.
On the internal level, you look at things like...
Purposes: what he wants
Methods: the ways, approaches he takes to get what he wants
Evaluations: how he judges his progress, situations, himself, other people
Dramatic function: which of the unique functions listed in the article on Archetypal Characters (on the Write a Novel page) does this character possess?
On an external level, you look at things like...
Physical traits and appearance, and how they affect his personality. Was he always handsome or ugly, athletic or awkward etc.? When was he born?
Intelligence, talents, limitations, weaknesses.
Interests & hobbies.
Cultural background, education, profession, experiences.
For each of these characteristics, do some brainstorming. Try to come up with a list of possibilities so you can choose what most appeals to you and what makes sense.
Often it helps to find one or two real life people you can imagine fitting the role of your character, and then give your character a combination of their traits.
Keep in mind that anyone can be an antagonist. Anyone can do evil in the right situation or circumstances, including those you might least suspect.
If all else fails, let him be a bit of a mystery for a while. You may discover more about the character during the writing process.