It is far too simple to say that Cromwell is regarded as a hero in Britain. Some admire him as a defender of Parliament, a supporter of religious toleration and an enemy of the monarchy; others dismiss him as a military dictator, a religious bigot and an enemy of the monarchy. Those who are most embarassed by his conduct in Ireland tend to be those who admire the other aspects of his career. The (unofficial) celebrations in 1999 to mark the 400th anniversary of his birth were rather muted.
It is also far too simple to say that ‘he was just a crazy Puritan who was hell bent on destroying the Irish and the Scots’. Although he was personally unpopular in Scotland, his attitudes to the Scots were very different from those he held regarding the native Irish. To him, the Scots, as doctrinaire Presbyterians, were just misguided Protestants who happened to prefer Charles Stuart (Charles II). His aim was to integrate them into a unified British state.
The invasion of Ireland was mainly revenge for the ‘massacres’ of 1641 and a pre-emptive strike against a perceived threat from a possible invasion of England by the Irish. Of course both of these assumptions were dubious in the extreme. The reports which had reached England during the Irish Rebellion had mostly been invented. Such is the danger of relying on second-hand reports of military attrocities. The possibility that Charles Stuart might use Ireland as the launch pad for an invasion of England was always unlikely, but remember that this was just what his brother would try to do in 1689. The fear that Charles I might link up with the Irish rebels had run deep within England throughout the 1640s.
The general view among modern non-Irish historians is that Cromwell’s conduct during the Irish campaign, with the possible exception of Drogheda, moreorless conformed to contemporary conventions of warfare. All seventeenth-century wars were brutal affairs.