Sadly Oliver Cromwell's reputation is tarnished because of his activities in Ireland. There are many who would deny that he deserves that reputation - The Cromwell Association website states - "Biographers of Cromwell have differed on this subject and the truth of what happened is often obscured by myth and legend. It served the interests of both sides at the time to exaggerate the outcomes of Cromwell’s Irish Campaign, and the axiom that truth is the first casualty of war was as applicable in the 17th century as in the 21st."
Whatever the truth about Ireland, He was a key player in the struggle against Charles I's attempts to move England into being an absolute monarchy - as was the 'model' in other European countries. As Professor John Morrill has written - “Cromwell was a man deeply committed to religious liberty and who tried hard to make government accountable to the people. However, like all rulers, he had to deal with real threats, which often resulted in tough action. If Cromwell had held free elections, as groups such as the Levellers desired, the majority of people would have voted for the restoration of the monarchy. Cromwell wanted to teach people the responsibilities of liberty so that they could then be trusted to exercise it properly.”
My recent post on Cromwell - with links to books can be found here
There is a further Washminster post - Oliver Cromwell