Considering the revolt of the Netherlands lasted 80 years, this varied greatly by period. Regents such as Mary of Hungary was able to increase the wealth of the region through trade, although she was over-ridden by Charles V who captured and executed leaders of the Ghent revolt to discourage further objections to higher taxation.

Margaret of Parma was a weak regent, which saw the rise of further objections to taxation and the rise of Calvinism. She also saw a power struggle between herself, the clerics, the magnates, the council of state, and Protestants. Her alarming letters, warning of hundreds of thousands of rebels, led to Philip II sending the Duke of Alba at the head of tens of thousands of Spanish troops.

Of course, the reign of the Duke of Alba itself started with the Council of Blood that saw strong repression of rebels, both Catholic and Protestant. Most importantly, those executed include Count of Egmont and Philip de Montmorency, both heroes of Philip II's army. Instead of pacifying the region, this fortified the belief of local nobility that their king was out for blood and revenge. Further, the presence of such a large body of troops led to increased tax and burden to local towns. The Duke of Alba was mostly victorious in battle, but the locals increasingly turned against him.

His replacement, Don Luis de Zuniga y Requesens preferred compromise. He recommended amnesty to all except the most ardent heretics. He led armies to victory in battle. However, when money failed to come from Spain, he was forced to negotiate and worse some of his troops mutinied and sacked towns.

King Philip II's illegitimate brother Don Juan of Austria, fresh off his great victory at Lepanto, was sent to the low countries to be regent, however he died and was replaced by Alessandro Farnese, son of Margaret of Parma. Farnese was able to rally the catholic rulers, mostly in the south, behind him. With local support he was able to win back cities that had been lost: Tournei, Maastricht, Breda, Bruges, Ghent, opened their gates to him rather than risk a protracted and bloody siege. He laid siege to Antwerp and won that city.

Were the Spanish the only ones who tortured and murdered dissenters? Not even close. As an example, Calvinists tortured and murdered Catholics in Brielle. Watergeuzens captured civilians and sold them to Mediterranean slavery.

This is a very big topic that is best answered if you specialize your question to certain periods, groups, or regions.

Lastly, you have to keep in mind that the first half of the conflict saw Spanish troops largely winning field engagements, as they had much higher quality troops. Their main challenge was money and local politics. This results in sackings and mutinies. This is not to say that the other side is innocent, but this impacts portrayal especially in English language historiography.

Edit: if you can read only one paper on this, I highly recommend: G. Parker, "Why Did the Dutch Revolt Last Eighty Years?," Trans. Royal Historical Society, vol. 26, December 1976. It covers the outbreak and development really well, and in good context.

There wasn't really a "Spanish occupation" akin to the Nazi occupation where "the Spanish" took over the country. The Dutch provinces, at the time, were simply another part of Philip II's domain, and when unrest was on the rise he used Spanish and Italian troops to try and restore order.

The Dutch revolt was one of open resistance, mostly by defiant cities, rather than a guerilla campaign or insurrection. Only at sea did privateers such as the Watergeuzen and the Dunkirkers wage irregular war on eachother's merchant shipping, but that was pretty much par for the course in those days. On land, it was a tale of siege and counter-siege, not of the Resistance skulking about in the dark.

Now, these sieges and counter-sieges could be very bloody. There were a number of violent massacres enacted by both sides, which did include mass-executions on occasion. On the Dutch side the massacre of the Martyrs of Gorkum is particularly infamous. u/Itsalrightwithme referred to this incident when mentioning the killings of Catholics at Brielle/Den Briel.

In typical Dutch histories, the command of the duke of Alba is considered infamously violent and bloody. The duke is depicted as a bloody tyrant who ordered massacres at Mechelen, Naarden and Zutphen, typically called "bloodbaths" in Dutch.

Now, Spanish troops in the Low Countries regularly sacked cities they captured, but that was in part because they were perpetually underpaid and mutinous. This was the case at Antwerpen, and at Haarlem, where the Spanish commander proved unable to honour his agreement that the city would not be sacked. (Though Alba did deliberately execute the entire 2000 strong garrison at Haarlem.)

At the other sackings mentioned, however, Alba deliberately instigated massacres as a terror-tactic. In a letter to Philip II he boasted that "Not a man born escaped" the sack of Naarden. His bloody sack of Zutphen indeed succeeded in intimidating many other settlements into surrender, though the massacre at Naarden had the opposite effect.

It should be noted that the Dutch and Spanish depictions of these events do differ in some aspects: the Spanish claim that the Dutch civilians never offered surrender at Naarden, whereas contemporary Dutch histories claim the massacre came after the town had opened its gates in what they call an example of Spanish treachery.

The Dutch used events like these heavily for propaganda purposes throughout their history, lending their cause an air of righteousness. To illustrate: the later 17th century historian P.C. Hooft accused the Spanish of practising cannibalism at Naarden.

Then again, cardinal de Granvelle did think call Alba's use of violence extreme, calling it "counterproductive" in one of his letters. Eventually, after many complains and limited results, the duke of Alba was recalled and replaced.

The Dutch also didn't depict other Spanish commanders in the same light as Alba. Alessandro Farnese, the duke of Parma, for example, has a quite favourable reputation in Dutch history books, despite coming much closer than Alba did to stomping out the Dutch revolt.