I can't speak thoroughly to the continent, but I can give an account of the actions of Cromwell that have given him the label of a tyrant. Hopefully someone who is more of an expert on seventeenth century Europe can provide a counter to this to help analyze whether Cromwell was an exception or simply another European monarch without the title.

Execution of civilians – At Drogheda and Wexford, Cromwell is said to have caused the slaughter of thousands of Irish civilians. There are problems with this interpretation, as women and children don’t appear to have been victims in the way the myths indicate (Gaunt, Oliver Cromwell, pp. 82-86). He did, however, decide to put all the soldiers to the sword in a means of terror, and as punishment for the massacres of 1641-1642 (The reports of the 1641-1642 were greatly exaggerated, but believed by Cromwell at the time – Canny, Making Ireland British). The slaughter at Wexford was blamed on miscommunications between the Irish commander and Cromwell, but Cromwell condoned the massacre after the fact (Gaunt, Cromwell, p. 82-83; Coward, Oliver Cromwell, pp. 72-74).

Mass death of Irish – I wrote a piece here on the death toll in Ireland during the period from 1640-1660. Some 300,000 people died due to famine, disease, war, and deportation (Gentiles, The English Revolution and the Wars in the Three Kingdoms, table 15.1). It’s hard to argue that Cromwell was personally responsible for all these deaths and deportations. However, one of the key aspects of early modern Kingship was the maintenance and protection of a King’s people. If Cromwell is being considered as the absolute ruler of the British Isles (which he was by conquest), then he had a duty to protect his Irish subjects. Proclamations regarding the price of grain, forbidding exportation, attempts to control the spread of disease abound in the Tudor and Early Stuart years (Hughes & Larkin, Royal Proclamations). While Cromwell was not personally responsible for their deaths, he certainly held the Irish in poor regard, condoned their awful treatment, and continued to appoint military leaders in Ireland who pursued harsh policies against the Irish. Also see my response on Cromwell’s Campaign in Ireland.

Mass deportation and “enslavement” of Irish and Royalists – This one is tricky because an enormous portion of the source material was destroyed in the Battle of Dublin in 1922. From the extracted materials published by Aubrey Gwynn and anecdotal material at the time, we can estimate that about 50,000 Irish were deported out of Ireland, and the vast majority of these ended up in the West Indies working on plantations (Gwynn, “Cromwell's Policy of Transportation” Parts 1 & 2, (1930) Studies: An Irish Quarterly Review; Lenihan "War and Population, 1649-1652" (Irish Economic and Social History, (1997), pp. 1-21). Cromwell was certainly responsible for the deportation of Royalist rebels from England following Penruddock’s rising in 1655 (Abbott, ed., The Writings and Speeches of Oliver Cromwell, 4 volumes (1937-1947), 3:720; Diary of Thomas Burton, 4 volumes (1828) 4:255-273; Hutton, The British Republic, second edition, 67-69). Most scholars, myself included, would contest the authors of more popular historical works that neither Cromwell nor any of the men involved enslaved the Irish (Eg. O'Callaghan, To Hell or Barbados). The prisoners and deported were sold as servants for terms of four to ten years. However, for the men who petitioned Parliament in 1659 that they had been sold into slavery, a lifetime banishment from the British Isles accompanied their servitude. No matter the term of labor, all of the above men were seized from their homes, forced to labor in the deadly West Indies against their consent, and all of these men were punished without trial.

Dissolving/coercing a Representative body by force – So there are a few examples of this. Pride’s Purge in 1648/9 was a coup led initially by Thoms Pride that imprisoned about two dozen MP’s and forbade by force the admittance of several dozen more into the House of Commons (Gardiner, History of the Great Civil War, Vol. 4, Ch. 68; Underdown, Pride’s Purge). Cromwell was not present in the planning or execution of this plot, but upon his arrival on December 7, he took charge and approved of the actions. It was largely through Cromwell’s guidance and force (both personal and militarily) that the Commons set up the High Court of Justice that tried Charles I (This is a disputed point with S. Kelsey that I will not get into here). Cromwell later did lead a military coup that dissolved the Rump Parliament in 1653 by force. Cromwell, frustrated with the lack of progress that the Commons was making, brought his army to Westminster, entered the chamber, and informed the Parliament that they were dissolved (The Rump may not have been a representative government, but it was more representative than the Protectorate that followed). After the dissolution of the Rump, Cromwell became, in essence, the central political figure wielding supreme executive power (Titles like Dictator or King are debated, but the reality of Cromwell’s power was not).

Religious Intolerance – This was standard procedure for most European Kings in the seventeenth century, but his deprivation of Catholics in Ireland to their lands and rights caused greater internal conflict, and further destruction of both lives and property. This was not unique to Cromwell, but certainly was part and parcel of his rule over the British Isles.

Above are the main elements of Cromwell’s rule that historians use to characterize him as a tyrant. While, again, I am not an expert in European governments, I know that Cromwell was unique in British history for his almost constantly resorting to force to ensure he got his way. James I and Charles I dissolved parliaments, but never at the end of a musket. Generals and Kings had put garrisons to the sword, but this almost never included civilians or loyal subjects to the crown. That Cromwell punished “offenders” without trial or attainder (indeed, it was by declaration) was a key element in accusing Charles I of tyranny in the case of Ship Money (Halliday, Habeas Corpus). Their penal servitude without trial was unheard of. Even in the only early modern case of Englishmen serving as slaves, the convicts were still given some due process (Davies, “Slavery and Protector Somerset; the Vagrancy Act of 1547” Ec. Hist. Rev., (1966) pp. 533-549). In compiling this list, I do subtly try and indicate that Cromwell did exercise tyrannical rule in specific circumstances. However, I do not know if he is unique if compared to the rest of the European continent.