Q. Why did God hate Esau even before he was born?
In response to your question, please see this post:
In that post I note specifically that while in Romans “Paul quotes the statement from Malachi, ‘Jacob I loved, but Esau I hated,’ we need to appreciate that the Hebrew language uses the term ‘hated’ in contexts like this to refer to a son or wife who is not favored, by contrast with one who is favored. The meaning is, ‘I favored Jacob, but I did not favor Esau.'” Malachi’s statement, and Paul’s quotation of it, reflect this usage. So God did not actually “hate” Esau. Rather, he chose Jacob instead to continue the covenant line.