Alright, so the question of Drogheda and Wexford and Cromwell's conduct there has been a fraught one for quite literally centuries, on one hand you have a tradition of Irish nationalists who put massive emphasis on Cromwell's ferocity and violations of the rules of war, and on the other hand you have historians arguing that holding Cromwell to such standards is unfair precisely because of precedents like Magdeburg.

I'm sure I come across as biased here, but I want to put emphasis on something else which tends to be evaded in this debate, perhaps deliberately, within the context of the wars that were happening in Britain, the sack of Drogheda in particular was really unprecedented and a major break from the conduct of war that Cromwell and the New Model Army had typically taken in the past. Sieges and battles during the civil wars in England and Scotland generally did not end with mass bloodletting and no quarter given, there were expectations of conduct that were mostly held and breaking that conduct could end up handing the opponent a propaganda coup instead, for example the Royalist commander, Prince Rupert, was pilloried by Parliamentarian Pamphleteers for his supposedly brutal conduct in and around Birmingham, and in that case the amount of dead was utterly paltry compared to Drogheda and the controversy mostly revolved around looting and pillaging. It's also worth noting that Rupert had grown up with the more brutal tendencies of continental warfare and tended to be seen as hot headed in general by the standards of war in Britain. On the whole, probably the closest analogue to what happened in Ireland in England was the final siege of Basing House, and this offers some interesting insights. Again the amount of people actually killed here was way less than Drogheda, but the Parliamentarians had lost a lot of men in assaults at this location in the past which probably built up a lot of resentment, as such quarter was already contentious if they continued to resist, but moreover Basing House came to be seen as a locus of specifically Catholic resistance within England, 'A nest of Romanists' as they were seen which had a consequence of greater brutality used against them compared to elsewhere as quoted from a contemporary newspaper:

"The enemy, for aught I can learn, desired no quarter, and I believe that they had but little offered them. You must remember what they were: they were most of them Papists; therefore our muskets and our swords did show but little compassion, and this house being at length subdued, did satisfy for her treason and rebellion by the blood of the offenders"

Anyway, I'm not going to go into major detail about Drogheda since it is an seriously complex topic but basic gist I'm trying to get across here is that the parliamentarian forces really did have different, much lower standards that they applied in Ireland compared to how they behaved in England because of factors like religious prejudice, and in my view this is a much more relevant part of the conversation than whatever was happening in Germany at the time (even so, the Sack of Magdeburg is a poor thing to lean on to defend Cromwell since it was a PR catastrophe for the Catholic side of the conflict that was widely perceived to be a ghastly violation of the rules of war at the time, the Protestants made full use of it in their propaganda to convince otherwise neutral princes to take their side lest the same thing happen to them, which likely influenced events negatively against the Catholic League, especially at Breitenfield later that year). The core of this was anti-Catholic sentiment that meant that much more brutal actions were deemed acceptable in Ireland than would be allowed in England, at Drogheda Cromwell justified his actions and the actions of his troops as both retribution for the 1641 rebellion (at the time English propagandists had vastly inflated the deaths in this event, but this was already a poor justification since a lot of the troops the Parliamentarians were fighting in Drogheda were English Royalists and protestants who had nothing to do with 1641) and as a specific example of what would happen to other cities that did not surrender to him (this actually worked, to some extent, after news of Drogheda spread many other towns put up no resistance). Already in 1644 parliament had declared that no quarter would be given to Irish troops they found in Britain (and they did act upon this, though when Irish troops were summarily executed Prince Rupert executed his own prisoners in retaliation, again this resulted in pamphleteer fury that he would value English lives as much as Irish), while the conduct of the parliamentarian campaign in Ireland reached sickening levels of violence and destruction, especially after Cromwell left and the campaign devolved into guerrilla warfare, killing somewhere between 30-70% of the country. The religious and ethnic prejudices of the parliamentarians almost certainly resulted in a much more brutal treatment of Ireland compared to England and Drogheda was emblematic of that.

If I can recommend a book on this I'll say pick up 'God's Executioner: Oliver Cromwell and the Conquest of Ireland' by Micheál Ó Siochrú.