Charles the First - What a Big Deal?

As a foreigner, I don't know anything about Charles the first having not grown up in this country. And it struck me that what a big deal it was to kill the king off the back of a trial. Oliver Cromwell is a name that is very much part of the decision to bring him to trial and have him execute. The story gets quite gruesome when we talk about what happened to him and the others some 30 years later after the death.