Here's another picture of him playfighting in the museum: https://ohnotheydidnt.livejournal.com/76938647.html
He's off his hinges.
My 26 year old cousin was playfighting with his father at my Granda's funeral, right at the graveside. He's not off his hinges. He's just a prick who had no real feeling for the situation he was in and a self centred twat who doesn't really consider the thoughts of others.
I visit these sorts of places, historical museums, WW1 and WW2 battle fields and graveyards, war memorials etc and see young people behaving like idiots all the time. It's just what some young people do. Bieber isn't a kid, but he isn't exactly a grown up either. He also lives in a bubble where people fawn and pander to him and that is his reality.
None of this is to make excuses for him or any other young idiot, more that I'm not sure why so many people are so surprised that these sorts of things happen. I don't think he was being offensive, he was just being a twat.
Plus...oh, what's that? Oh he has a tour to promote and an album to sell? Wow, I can't imagine why his press office would want him to be in the media spotlight.