What would happen if Prince Harry decided to Kill everyone around himself + make him the Rightfull King. Legally could he be done for this?

The old monarks did that all of the time .

By today's standard you are describing an act of treason. Harry would be jailed for life.

Perhaps in the 1300's, this could have been done. but not in today's world.

No, its very naughty to kill, even royals. He would be given a padded cell to get well. Old ladies would write him love letters.

How in the world can 'murder' make anybody a 'rightful King'??

He would become king. The monarchy would be abolished. He would be prosecuted.

He couldn't, he would be arrested for Regicide. Being only a Prince, at the time of the murder.

Oh, that's old hat.

He would be deposed and gaoled. Parliament would choose another King. They've done it before.

1649 Charles I was beheaded, the monarchy was abolished and England became a Commonwealth.

1660 Charles II was invited to be King in a restored monarchy

1688 James II was deposed and his daughter Mary was declared Queen Her husband William was co-regnant

1701 Catholics or people married to Catholics were excluded from the the throne by the Act of Succession

1714 Queen Anne died and George of Hanover was declared to be King even though there were about 50 Stuarts with a stronger claim to the throne but were excluded by the Act of Succession

1936 King Edward VIII was forced to abdicate by Parliament in favour of his brother George VI

If Prince Harry murdered all those ahead of him, he would not become King as he would not be approved by Parliament. The crown would go to his uncle the Duke of York.

This is what you're sitting there thinking about

He would soil himself.