Sounds like weed and alcohol was not enough for Jon that night.
You have to assume he was heading out for strange and coke.
The thing is.... dude can keep a side piece and a apartment full of coke at all times if he wanted to, uber there whenever he wants. Maybe he is trying to leave new pussy and coke alone and he just got drunk at home, his fiancee of 15 years just got on his nerves so he got drunk and snapped.
Who the hell calls their girlfriend a "fiancee" for over a decade? Nobody plans a wedding for a decade. That is no finacee, that is a girlfriend or a baby's mamma. I imagine any woman that gets called a fiancee for an entire decade would be embarrassed as fuck.