I think it’s still worth trying to see if she really feels like she wanted you to be more concerned for her safety and maybe yelled at the guy or did she really want you to go full murder hobo on an innocent human.
However, the extreme mood shift and being abusive to you and her doing fuck all to break that cycle where she’s not being a dick to you every damn month, well I think that really needs to be addressed and you need to decide if you’re actually going to be okay with being treated badly every month for t he rest of your life.
The other elephant in the room is that do you want to spend the rest of your life with a person who’s feelings will turn on you the minute things get a little tough? I mean you not wanting to murder someone leads to her falling out of love with you that fast? Really man, take a good long look at your two years together, maybe hash it out with a friend and get a outside look at things to get a full picture of if this is really what you want for the rest of your life.