Slavery existed in its American colonies; Suriname and several Carribean islands.

Technically there was no slavery in the Dutch Republic proper. In practice it was tolerated, escaped slaves would be returned because otherwise they robbed their masters of property.

In the half century before 1776 less than 500 slaves came to the Republic. In 1776 it was decreed that slaves in the Republic had to be liberated after staying there for 6 months, further reducing this number.

After this early sign of opposition to slavery little happened for a long time. Only in the 1840s did several abolitionist groups emerge. These groups had hundreds of members and their petitions were usually signed by hundreds of people too - which seems minor in a country of 3-4 million.

Obviously people knew that slaves were treated poorly, but really caring about it is another matter. Uncle Tom's Cabin helped in this regard, making it more personal and less distant. British abolitionists were also influential and seen as an example, so some may have been familiar with their propaganda (like the infamous stacked slave ship). Some 11.000 Dutch sailors had been slaves themselves, i imagine that at least to them this was enlightening.

Pro-slavery rhetoric was all about money, aside from some early biblical references. One of the reasons why it took so long to abolish slavery is because the state couldn't afford to recompense the slave-owners during the 1840s and 1850s.

Something of note which i nearly forgot: In the Dutch East Indies a form of servitude had been introduced in 1830 known as the "cultivation system". It started out as a simple 20% tax on crops grown but became increasingly oppressive.

In the 1850s people became more aware of how the local population suffered under the system and after Multatuli published Max Havelaar about this in 1860 there was public outrage and the system was dismantled in the next decade - just like slavery was abolished only 3 years later.