The theme is apparently the oppression / murder of Catholics by Protestants under English (Protestant) rule. Cromwell was a Puritan leader instrumental in bringing down the Stuart monarchy in England, who at that time (1640s) were no longer Catholic, but were pushing the more Catholic like form of the Anglican church (though they were to reform members for some as well). I gather that the Stuarts inherited the crown when Elizabeth 1 died without an heir and Mary Queen of Scots' (yes, I now know that she's not Bloody Mary) son was the next heir. By the time of Cromwell there had been several Stuart kings who were locked in big struggles with Parliament over power, including church power.
The Puritans were the radicals, many wanting to do away with the monarchy, some were pure leftist zealots who, among other things, wanted to strip away all forms of ritual left from the split with Rome in favor of a "pure" religion. Doing a quick Google on Cromwell and Catholic I got a fairly good history here. I do know that Cromwell extended the civil war into Ireland and the identifying symbol of the Protestants, wearing orange (usually sashes), is still a central part of the symbolism in the Catholic/Protestant battles there.