I'd definitely invite somebody else over to be at the house with you when he comes to pick up his stuff. You don't know him well enough yet to know if he has any violent tendencies or not. If he does, this would be when he'd be most likely to use them. To play it safe, don't be home alone with him. That way he can't pick an argument with you either.
Just let him get his stuff and leave. Trying to get closure from an after break up talk rarely works. Plus, considering he's a selfish liar, he's likely to just manipulate the conversation to make you feel worse. You should really just try to avoid him as much as possible.
You might never know what his motives for doing the things he did were. You just have to come to accept that. All you need to know is that he's not worthy of your time or energy.
This guy is just a jackass. He's not good for anyone. If there was a mistake on your part, it was being too trusting and rushing into everything so quickly. You shouldn't have move in so quickly with a new relationship- nor believed everything he told you so soon. Trust needs to be earned and proven. He may have seemed great at first, but there was a lot you still didn't know about him. That is the risk you take when you jump into things so quickly. If you want to learn more about what to watch out for in the future, the book, "How to Avoid Falling in Love with a Jerk" by John Van Epp is a good one.
Go ahead and change the locks. Then just be glad that you didn't spend any more of your time on him.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy