In your example, the word it functions as a pronoun and therefore refers to what the subject said. It is needed after said in your sentence, but you should make the reference more clear. Here is an example: "Once I'm finished with him, he'll regret his comment."

Probably. 'Once' here stands for 'as soon as' , and 'it' is needed or there will be no object. I think this sentence means 'He will regret what he has said , as soon as I have finished with him'

Thank you for lots of help (^^)!

My interpretation of the sentence is "I'm going to make him pay for saying it" The writer wants the reader to understand that he has said something inappropriate. In this sentence "it" is the topic of the discussion. "it" relates to something specific that would offend her. If you remove the "it" topic it still clear that he said something offensive. but the focus is about what he said exactly and not about him speaking or He can be regretful for saying "it" but he can't be regretful just for saying So without the "it" it's a incomplete thought. Hope I helped and did not confuse you more. Jia You!!

You have proved that you have gained the upper hand in this relationship!