This serves to illustrate that history, although unchangeable, may be shown in different lights dependent on who is doing the telling of the story.
Amandistan, I'm not being confrontational towards you, or rubbishing what you have been told when I say this: you have been given one view of a huge slice of European history from one source. The view is highly subjective, (rightly or wrongly) biased, incomplete and very emotional.
Oliver Cromwell, be he saint or sinner, did not just decide one day to invade Ireland off the bat, persecute the Catholic people that he found there and set the course for the suppression and denigration of the Irish people for centuries. It was all part of a much, much more complicated and turbulent period in European history; I say European because it involved the quarrels, religious intolerances, successions and political manoeuvrings of each of the major (and several of the minor) states whose influence was waxing and waning in Europe (and also in the New World) at the time.
If you would like to read further into this issue, I can suggest Neal Stephenson's Quicksilver trilogy, which is a historical fiction, accurate where points of historical fact occur, that will keep you going for months, showing you in minute detail how turbulent, violent and confusing those times were.