Let’s start by stating the obvious: yes, this is a hypothetical question. But what if Queen Elizabeth II actually killed someone? Or robbed a bank (can you imagine how surreal that scenario would be)? Or whatever. Can she be arrested?
Well, no. The Queen cannot be arrested in any case.
And this is why. (the article continues after the ad)
When someone breaks the law, a court case is opened. But in the United Kingdom, all court cases are “The Queen versus”. That means that the Crown itself is the prosecution judge thus she cannot be charged with crime because you cannot have Crown Vs Crown!
OK, Queen Elizabeth, you are free to break the law in each and every way you like.