I have read Antonia Frasers book on Cromwell and Trevor Royles Book on the civil war and also a couple of biographies on Charles the first and Charles the II. I feel that Charles I was a king that never really understood how england was governed, the role that parlament played and that he just couldn't decide for himself. In that he was like his father, James I, but he lacked james ability to get along with people. He rubbed everyone the wrong way and so started a civil war that noone really wanted in the beginning. Cromwell did not start it IMO, he just capitalized of it. Cromwell was the military leader of the rebellion but never in the beginning the one with the voice people listened to. When his ironsides won the war he did not want to replace the monarchy with democracy or rebublic or ruling by parlament or anything like it. He wanted a dictatorship disguised as something more respectable. So he called himself Protector and ruled by himself in what was, really a military dictatorship. So no i dont feel it was justified. You might say the beginning of the rebellion was justified because Charles I was an incompetent King. But the rest was just a waste of men property and money with the result of replacing a king with a Protector. No big difference really. Charles II understood how ro manipulate the parlament to do his bidding often. If you're looking for justified rebellion/revolutions then when they kicked out James II, brother of Charles II, and replaced with first William and Mary II and later Anne I , both of whom were daughters to James, that can be said to be OK. James II was also not a very capable king.