Well as ever Amanda, a small amount of research would have easily answered your question but no matter.
Q.1. Hero? I would say he isn't (and most historians agree). One of the most influential men in British history absolutely, but actually hes an incredibly dispersive figure. Hell, even Churchill considered him to be a Dictator.
Q.2. He hated Catholics? Well he certainly hated the Catholic Church which at the time was (and had been for hundreds of years) the most powerful/rich and authoritarian organization in the western world. Which in itself had caused multiple wars and the death of hundreds of thousands of people (not forgetting the persecution of women by the inquisition). Puritanisn was a reaction against this and at the time was seen almost as a working class revolution. Of course, they ended up as bad in some cases as the regime they sought to overthrow but thats what happens when you pitch one strand of religion against one another. Lets not forget, its all Christianity...
The Irish situation at the time was also in itself incredibly complex. Far from being a happy country they were at the time constantly at war with each other. The reason Cromwell was 'sent' there (this is an important thing to remember) was that an alliance had been formed that threatened to invade England. An alliance that had already massacred protestant settlers. Does this make it right? no of course not but again you need to look at the whole situation and not just one point of view. He did appear to hate the Irish its true. But this was purely down to their perceived support of the Holy Church of Rome who again were the richest and most powerful authoritarian regime of the time.
Did he effectively start an ethnic cleansing campaign? Yes. Is this taught in schools? Guess what...Yes! (Or at least it was when I studied history).
Cromwell was in some ways a very great man. The establishment of Parliament having primacy over a Regent is not to be underestimated, but he was also at times a vicious dictator. In essence, he was very much a man of his times and arguably was no better or worse than most other rulers of the time.
That said, to compare him to Hitler is utterly astonishing, and I would suggest rather insulting to all the people who were murdered in the name of National Socialism.
All of this info is easily available on line, in fact the wiki page on him is (in this case) pretty good. I'm assuming your thoughts on him are colored by your stay in Ireland? Well if so, I'd suggest that approaching history from only one point of view is extremely dangerous. A good (and more recent case) would be the Palestine/Israel question!
Oh and by the way, just in case you think I'm anti Irish or Catholic (I'm not), my family are Irish and Scottish Catholics, in fact my Mum is still practicing. I myself am not its true, but that's another story...
Here's the link to the wiki page. Have a read...
https://en.wikipedia.org/?title=Oliver_Cromwell