What would happen to the monarch if this happened? (UK)?

Probably the monarch would be removed by the police and taken to a secure location, a regency would be declared and there would probably be a motion to abolish the monarchy.

Less likely is the monarch would have the support of the police or army and would be able to rule under a form of martial dictatorship.

They would be declared insane, deposed and their successor appointed.

Wow - why not write a book. HM wouldn't get past the door into Parliament.

It would still be considered treason to defend oneself against a monarch who does that, she would cease being a constitutional monarch and become an absolute monarch.

It's not going to happen so get over it.

And if she used a tin of chicken soup to commit the murder and then washed her fingerprints off the tin with lemonade, on a blisteringly hot afternoon in the middle of winter. The world, as we know it, would end!

The masses would rise up and rid themselves of the oppressor. guaranteed.

This is fantasy nonsense.