A complex figure, far too complex to be summed up in either of the glib descriptions which you present. He was good, and he was bad, and, as much as anything else, he was misguided, just like most people are, and always have been. He merely had the power to show it in a way which most of us never will.
Irony abounds...
I think that it's highly questionable whether one can even very loosely consider the Protecorate campaigns in Ireland as "genocide", given the lack of apparent ethnic motivation. It was, first and foremost, a war of political ideology, Royalist against Parliamentarian. To a lesser degree, it was a conflict of religion, a coalition of Catholics and conservative Anglicans against one of radical Anglicans, Presbyterians and Independents. It may even be possibly be called a nationalist conflict, but that is still rather simplistic, given that many among the Irish had no desire to rid themselves of the Anglo-Scots Stewarts- that, in a large part, is later Nationalist revisionism.
Even to style it as "religious genocide" is questionable, given that it lacked the orchestrated mass-murder of civilians which is typical of such campaigns (not so on the part of the Irish Catholics, of course, who cheerfully butchered Protestant civilians throughout the conflicts). Indeed, the only such example I can think of on the Parliamentarian side is the Sack of Wexford, which was not ordered by Cromwell, nor had any explicit religous or religous motivation. If nothing else, to focus so exclusively on Cromwell's supression of Catholicism in Ireland fails to acknowledge the wider social and political situation of the era.
Again, it's a very complex issue, and such simplistic summaries help no-one.
I've always wondered why Cromwell has such a lingering infamy, when his contemporaries, not to mention those who followed, would do far worse and, at worst, be forgotten by history. I think, perhaps, that he's simply too modern for people to accept. Kings and emperors and lords may butcher and rape and pillage left, right and centre, with nothing to justify their actions but arrogance and power, but that's just history, that can be forgotten, even forgiven. But for a man to establish a military dictatorship as an attempt to realise an idelogical system? Well, now, that's politics, and we won't forgive that nearly so easily.