Yeah, that was cruel. It happens when someone feels hurt and angry enough to lash out, it can be a rush of power and relief to be able to hurt back, but, like the voicemail, it can't be deleted or forgotten.
Randall, like Kevin, has been able to get through many things in life with his charm. But that has also given him a big, honking blind spot to seeing things from others' points of view. Deja has called him on it at least once. His one flash of clarity came when he remembered the Jack and Rebecca fight and wouldn't escalate to Jack's level. But that may have made things worse by also not acknowledging that Beth had cause for feeling how she did.
While Beth is in the right about many, maybe even most, things, she is wrong about some things as well. She complained how she had "asked" when she needed a respite while William and Kevin were there. She chose to ask instead of take the lead (boy, was William right) and tell Randall it was happening, this was when, no negotiation, and let him handle things while she was gone without any criticism when she returned. They had a built-in babysitter with Kevin, it's not like the girls were going to die if they lived on pizza and cereal for a couple of days and no one did the laundry. She was caught between her need to take a break and her need to manage things. It's like when she woke up to micromanage and criticize Randall through the diaper change. She may be more like her mom than she knows.
It's how most marriages are. It's not always Team Wife or Team Husband. Both went down the path to where they are now.