On January 30 1649 Charles I was executed by being beheaded and since then there has been a persistent tendency to make his death somehow "progressive". This is done by either excusing his execution, i.e., finding the reason solely or largely in the Kings own behavior or by arguing that since in the long term all of this help to lead to constitutional government in England it was somehow a "good" thing.
To talk about the Trial on the most basic level of procedure; lets see. The House of Commons was purged before the trial and the House of Lords was not involved. The verdict was decided before hand and the presence of the army ensured a "correct" verdict. John Cooke basically functioned as head prosecutor in a show trial. This trial was nothing more than a dress rehearsal for the Great Purge trials in Russia. Cooke serving in a position similar to Vyshinsky the infamous Chief Prosecutor in the Moscow Trials.
Cromwell and his associates had purged Parliament before the proceedings and, along with the army, were ever present during the Trial and the verdict was discussed and decided before even th first words were spoken during it. The whole thing was a ludicrous, murderous farce. And it appears that several of the "commissioners" were directly coerced into both the guilty verdicts and signing the death warrant. Of course the process by which those "commissioners" were appointed was frankly illegal. (The lords being dismissed by the army and the House of Commons being purged, of at least 1 / 2 its members). Over half of the "commissioners" didn't even bother to show for the trial, even though they were pre-selected by Cromwell and his cronies. In comparison the Trial of Louis XVI during the French Revolution was a model of fairness.
The charges against Charles were shall we say "defective", although "shabby" and "contemptible" are better words. For example, Charles:
... out of a wicked design to erect and uphold in himself an unlimited and tyrannical power to rule according to his will, and to overthrow the rights and liberties of the people, yea, to take away and make void the foundations thereof, and of all redress and remedy of misgovernment, which by the fundamental constitutions of this kingdom were reserved on the people's behalf in the right and power of frequent and successive Parliaments, or national meetings in Council; he, the said Charles Stuart, for accomplishment of such his designs, and for the protecting of himself and his adherents in his and their wicked practices, to the same ends hath traitorously and maliciously levied war against the present Parliament, and the people therein represented, ... 1
All which wicked designs, wars, and evil practices of him, the said Charles Stuart, have been, and are carried on for the advancement and upholding of a personal interest of will, power, and pretended prerogative to himself and his family, against the public interest, common right, liberty, justice, and peace of the people of this nation, by and from whom he was entrusted as aforesaid.
By all which it appeareth that the said Charles Stuart hath been, and is the occasioner, author, and continuer of the said unnatural, cruel and bloody wars; and therein guilty of all the treasons, murders, rapines, burnings, spoils, desolations, damages and mischiefs to this nation, acted and committed in the said wars, or occasioned thereby.3
Civil War, Trevor Royle, Abacus, London, 2004.