This is the end of the game. This player needs a long ban. For a referee to have to run off the FOP as he fears for his safety is scary. This isn't some local park either, he is a level 3 referee. Horrendous.
The offside itself needs to be separated and looked in its own context. Its an horrendous error at this level of football. We're all capable of screwing up in the wrong circumstances and this is a bad one. The AR's must help here too. The ball was put in the back of the net before the whistle went which suggests to me that the ref has held the whistle because he knows it should be allowed (ignored the flag until the goal is scored) and imo that would be excellent refereeing if he then ran over to the AR, had the chat and then allowed the goal. Or, the ref doesn't even look over at the AR as he's not even considering an offside and then when he's seen the flag after the goal is scored the shock of it all has clouded his thinking? But there's a very high chance they're using buzzers at this level so that would be surprising.
Can't help but feel sorry for them.