The nature of who willed their deaths essentially.

Let's take Richard III as an example. Richard III is King, but Henry VII disputes it and claims himself the throne for himself. This is a war over who the correct claimant us, they fight a battle and Henry wins. Henry is now King and in the medieval eyes willed by God to be on the throne. Edward II likewise was over throne in his son's name and Henry VI murdered over Edward IV's claim. These wars of claims over titles fits well into the medieval mindset. Treason in these cases were if one party betrayed their oath to the King. Important, this was going back on your personal path to the King not the state as the state was part of and inseparable from the King's person.

When the decision is made to try Charles the Parliamentarians have a problem, as Charles says in his defense he can't betray himself, how can they possibly try him fir anything. He is the state and all the law courts exist as delegations of his power. So they have to create a seperate entity from the King that they can state he has betrayed, a new authority the courts authority comes from. They come up with the Commonweal or Common Wealth. It's not quite the creation of nation seperate from the King but is a construction of a community of the people and Lords of that signified a big change in the relationship with the monarch. The ruler was subject to the authority of the Commonwealth.