Historical events can’t be studied in isolation without the proper context.
Nothing is going to do justice to such a complicated event other than a lot of time spent doing research using various sources. I wouldn't say Cromwell is regarded as a hero, but rather that his historical importance can't be denied.
Cromwell and the Parliamentarians moved against an increasingly authoritarian King Charles I who sought to disregard English law and establish an absolute monarchy akin to what existed at the time in Spain and France, both Catholic countries and sworn enemies of England. This doctrine of The Divine Right of Kings is opposed to English Constitutionalism (Parliament) which is why the Parliament wouldn't take it sitting down.
By the 1630s and 1640s Anglicanism and Presbyterianism had become firmly established in England and Scotland. Anglicanism had rooted in Ireland, but not with great popularity; Presbyterianism in Ulster was more successful. What came with the more radical Puritanism of the Parliamentarians was an ingrained disdain for Charles I's policies, so there was very little support for his plans of grandeur for himself at the expense of everyone else. This built a firm enough platform for Cromwell to challenge him, and the monarchy itself, successfully. Charles I's reign had already de-stablilised England, Scotland and Ireland for various reasons and there was widespread discontent.
In Ireland, the Old English (settlers from the Norman period) and Irish Gaels took the side of Charles I and formed a "Catholic Alliance" against Cromwell and the Parliamentarians (The Confederation of Kilkenny). Why? Because of their disdain for the radical Puritanism of Cromwell. That, in combination with the recent massacre of Protestants in Ulster in 1641, convinced Cromwell that Ireland, in particular, posed an equal threat to the English Parliament on par with Charles I. So, the New Model Army landed at Ringsend in Dublin on 15 August, 1649, to lead the offensive. They might well have been signing Vengeance with lyrical modifications, given Cromwell's personal desire to avenge the events of 1641 in Ulster.
Cromwell, no doubt, was heavy-handed by today's standards. The idea was that if a town surrendered to the military victor, then its inhabitants were spared. If they didn’t surrender, then they would be massacred. Cromwell massacred those in Drogheda, Wexford, Waterford and Clonmel, but spared those in Kilkenny, New Ross and Carlow who agreed to his terms.
Cromwell’s effect in Ireland was to confiscate land held by Catholics and ban the religion outright. The religion was not only sympathetic to Charles I (who was not Catholic by the way, but wanted to use Catholics in Ireland to support his vision), but also to the monarchies in Spain and France. Religious wars were standard behaviour for the time; The Thirty Years’ War had only recently finished. The Treaty of Augsburg, signed after the war, paved the way for religious harmony, if you could call it such, by expecting citizens to adopt the same religion as the ruler. It accepted the permanent division of Christendom within the Holy Roman Empire and eventually beyond...
Cuius regio, eius religio
Only Ireland didn’t follow the example of converting to the religion of the victor.
