It's not really about if your a royalist or not, myself didn't mind the Queen, but wouldn't lose any sleep if I went to bed and we were a republic,in the morning .
It's about respect.
You should respect someone has died, and even if you had a view on them just keep it yourself keep quite It's not hard.
It's about respect for the club who will get the stick for any bad publicity.
It about respect for your fellow fans who might want to show it even if you dont.
would you start to boo outside a church if someone you disliked died , because you would look a right knob.
Why do it at a ground in front of the national TV/press?
In modern life there is always somebody trying to be the odd one out, look at me ect.
It doesn't make you a bootlicker to have some manners and self respect.