ARGUMENT 1: HE DESERVED TO DIE
Let's be clear, this isn't a debate about the rights and wrongs of capital punishment. We're not asking whether it was morally wrong to put Charles I to death. We're asking whether, in the context of the 17th Century, and the prevailing opinions of his peers, his execution was justified.
It's a misconception that Charles was killed simply because he lost the English Civil War against Parliament. A simple glance at the dates will prove this: Charles surrendered in 1646, but wasn't executed until 1649. In those intervening years, he was given plenty of room to manoeuvre, as it was generally assumed that the nation needed a king. A king with new limits imposed on him, yes, but with the crown still on his head (and his head still attached to his neck).
In other words, Charles could very easily have played things straight and negotiated a new deal with Parliament. He could have preserved the monarchy and his own life. Instead, he decided to backstab and betray the very people he should have negotiated with, ruining the post-war peace process in the process.
Remember that thousands of Britons had died by this point, and the nation had been ripped apart. Instead of trying to heal his realm and maintain his own position, Charles engaged in the worst kind of double-dealing and political skulduggery, making a secret pact with Scottish forces. This was known as the "Engagement", in which a Scottish faction agreed to invade England and help restore Charles to the throne by force.
Think about this for a moment. One catastrophic war had just ended, and Charles willingly instigated a new one. This second Civil War was quickly brought to an end, but the damage to Charles's reputation was done. Cromwell, who until this point was willing to make a deal with the king, described it as a "prodigious treason". Charles had proven that he couldn't be dealt with in a reasonable way, and that he was willing to cause immense bloodshed rather than forge a new society with Cromwell.
This is why he was rightly condemned as a traitor to his own nation, and was made to face the executioner's blade. There was simply no other way for the country to proceed, because Charles himself had given his captors no other choice.
ARGUMENT 2: HE DID NOT DESERVE TO DIE
If it was so necessary and obvious that Charles had to die, why was the lead-up to it so reluctant and shambolic? It's a simplistic falsehood to claim that Charles was regarded by Parliament as "beyond the pale" after he conspired with the Scots to instigate the second Civil War. In fact, the prevailing establishment was very ready to resume negotiations with the king even after this setback. In December 1648, they voted overwhelmingly to carry on dealing with Charles and iron out their difficulties.
And that's what would have happened if Cromwell and his military cronies didn't force Parliament's hand. How? By ruthlessly removing any pro-Charles voices. As Professor Ann Hughes, an expert in Civil War-era England, puts it: "Over half of those sitting in the House of Commons in December 1648 had to be purged before the trial of the king could be undertaken - and this was of course a parliament from which royalist sympathisers had long been dismissed."
In what basically amounted to a military coup, Cromwell's soldiers literally stood outside the House of Commons and arrested any MPs who were in favour of continuing to negotiate with the king. This left a rag-tag "Rump Parliament" to do Cromwell's bidding. Hardly a democratic process, then. Meanwhile, the House of Lords rejected the very idea of putting the king on trial, but the Rump Parliament bulldozed ahead anyway, much to the horror of many grandees.
In fact, as Professor Ann Hughes says, "Barely half of the men nominated to the High Court of Justice to try the king actually attended its proceedings", and "some later claiming undue pressure, especially from Oliver Cromwell. One member of the Rump, Thomas Hoyle, committed suicide on the anniversary of Charles' execution in 1650, while the death the same year of another, Rowland Wilson, was attributed to melancholy and guilt."
Does this sound like a triumphant show of justice, agreed upon by all right-thinking people in the land? No. It sounds like what it was: an act of pure vengeance forced through by Cromwell and others who despised the king. The fact that the judge who presided over Charles's trial had to wear a bulletproof hat, and armour under his judge's robes, reveals just how unpopular the whole process was.
And as one contemporary commentator wrote, no monarch "ever left the world with more sorrow: women miscarried, men fell into melancholy". While it's true that Charles behaved with clumsy arrogance, and was a poor strategist, his death was not inevitable, and the true traitor to the nation were surely those who had demanded the death of their king.