AT you are a wordsmith and understand the meaning of words even in the " vernacular...." I'll f...ing kill you " said generically is very different to the specifically said " I will kill you if you leave me".It can be a vernacular thing. Here ‘I will F’ing kill you’ is commonly used in a jovial way with mates. Rarely would it make it to law here as a genuine threat (on its own anyway).
Can’t speak for your mate of course but I can see how some things are lost in translation.
I'm sorry but there is very little room for doubt in my mind