Max, you’re getting some good comments here. Of course it’s important to know WHAT the problem is. Have you had a conversation with the client? Is it your editing, or the footage that’s the problem? If it’s that they found someone cheaper, or a friend to edit the footage, give them a price that you can live with to give them the footage. Then if you wish, don’t work for them again.
No matter what, always be professional. It doesn’t matter that your client may not be acting in a professional manner. YOU need to be the professional in this scenario. Good Luck.