content: EDIT: I missed out the treason laws @Ulluses mentioned, but it's still an interesting matter. He wouldn't become King, the next in line is their son Charles (as he's got the blood of the Queen which is more 'royal' than Philip's). But let's suppose Charles is pissed off at ol' Lizzy living too long, and decides to murder her to take the throne. First of all, a very interesting thing about the reigning monarch is that they are basically above the law. You're not allowed to arrest them. At all. They're immune. They don't have to excuse themselves as they haven't broken any laws. The laws of government apply to the monarch's subjects, not the monarch. Also, no arrest can be made in the presence of the Queen/King or even near a royal palace if they're living there.  The loophole here is that Charles doesn't instantly become the King after his mother dies. If he somehow murdered her without anyone finding out until after he becomes King, he'd be good legally (but in that case public outrage would most likely call for the government sacking him, or the monarchy as a whole, assuming it isn't covered up). If he murdered her and was instantly found out, he'd probably end up in jail and the next in line (William) would take the throne. It is somewhat frightening that the Queen could become a serial killer and still be in the clear in the eyes of the law, but realistically it'd never happen, they'd change the laws/make exceptions or even abolish the monarchy. Not that she'd really need to break the law as I'm fairly certain she has a pretty good deal going on. Fun little tidbit: Not only is the monarch immune from all laws, but members of the royal household (relatives, blood or married/adopted) are immune from arrest in civil proceedings (but not criminal).