I often see an argument pop up that we can't (or shouldn't) condemn slavers because, in the context of the moral norms of their own society, they weren't doing anything wrong. However, there seems to be much to suggest this wasn't the case. I remember hearing a long time ago that the loss of the American colonies led Britain to look for a reason that God had decided to punish them, and what they settled on was the sinfulness of the institution of slavery. If slavery really wasn't seen as a bad thing by the majority, then that seems like a strange conclusion to reach. I know there were also many slavers who saw slavery as a bad thing that should be abolished, like Thomas Jefferson for example, despite the fact that they themselves owned slaves, proving that a society can have slaves without morally approving of having them. Queen Elizabeth I, who funded the pirate John Hawkins when he became the first Englishman to run the triangular trade from Africa-America-England, is similarly said to have despised slavery and to have cried when she heard England was trading in slaves. She believed that capturing Africans against their will 'would be detestable and call down the vengeance of Heaven upon the undertakers', even though she approved of slave trading for financial reasons eventually. France had its freedom principle and repeatedly claimed that any slave who stepped onto French soil would be immediately freed, and they were proud of that even if it wasn't actually a legal reality. In England in 1587, a priest and chronicler named William Harrison said: 'For slaves and bondmen, we have none… if any come hither from other realms, so soon as they set foot on land they become so free of condition as their masters.' It seems obvious then that the idea of having no slaves or slavery was a point of pride for the English and the French, despite the legal realities of the time. Even going back to the crusades we can see that a contributing factor that led to the second crusade was the enslavement of Christians in Edessa.
So, my question is, with all of the above evidence that slavery was widely seen as an unethical institution for centuries before its abolition, can it really be true that, prior to its abolition, slavery was seen as morally correct by the majority?
So there's a couple things here to unpack. First, your question treats slavery as a constant concept in all areas over a 500+ year time span, which it simply was not. Slavery to a 16th century Englishman would be a known concept for sure, but he would conjure a much different picture than a planter in Barbados in the 17th century and neither would match the understanding of an 18th/19th century Virginian planter. All of this goes back prior to my ability to convey in depth, but the feudal/serf system ended which essentially ended slavery in England. Several holdover labor systems remained, however, that bonded worker to master in a fashion we would consider absurd today, such as villeinage in which a person is bonded to the land itself and applied hereditarily, meaning you could be bonded from birth. Manumission wasn't too uncommon, and some of these were voluntary - to many it was preferential to being a non-landed worker or part of the lowest free class, a vagabond. They enjoyed more liberty and protection than we think of today in reference to a bonded larborer, but they were not legally permitted to leave without the landowners permission and worked for their benefit. And they were legal chattel, but again that conjures a misrepresentation due to the later development of "chattel slavery" in the Atlantic. The point here is that bondsmen were an understood part of English society and served to protect much of society as a whole from degrading to a point of poverty and destitution. It was needed for stability and wasn't what they thought of as slavery, which was not only terrifying but repulsive to every Englishman. So what was that? Galley slavery was certainly a concern of English sailors, who may be pressed into service for a number of years on a foreign crew, and it represented England's conceptualization of slavery. In fact thousands and thousands of Englishman (and women/children) had suffered this fate, often being captured by Barbary pirates. For most this life was short, often dying from starvation, dehydration, or just being worked to death. Sleep wasn't more than an hour or two per day and was done in place and in shifts to prevent ever being without oarsmen. It was a short and brutal life for most, and this was the idea of slavery to a 16th century Englishman. Even so it was almost always a timed indenture that some even voluntarily entered into, though many did not ever live to end the term. In that same century, in reference to a slave brought from Russia to England, the court decided "England was too pure an air for slaves to breathe in." This was during the Elizabethian Era, so it is somewhat reflective of the Queen's stance.
Meanwhile, however, the Spanish (and Portuguese) had already embraced the concept and started the Atlantic slave trade beginning with a Papal Bull issued in the middle of the 15th century granting permission to capture and enslave any "heathens" and to put them and their lands to proper use, as dictated for man to do under his Covenant with God in the Bible. This all coincided with their exploration and colonization of the Atlantic islands (Canary, Azores, etc). Soon they would be in the new world providing a market for enslaved laborers and massive bounties on the seas, ships laden with valuable laborers or chests of riches - both a desirable cargo. The English were also honing their colonization skills in Ireland most specifically and not until just before QE took the throne in the mid-1500s, and with folks like Humphrey Gilbert doing the dirty work. The Spanish and English had been fighting (on and off) for a while at this point, something that would continue in the same capacity for the next hundred years in the Atlantic World. After Hawkins made his raids, Sirs Frobishier and Drake, respectively, raided and took many enslaved souls just as Hawkins had; from the Portuguese and Spanish traders. Sir Walter Raleigh was the fourth of Elizabeth's "Sea Dogs" and his wife, Lady Raleigh, was gifted a group of enslaved Africans by Drake in the late 1500s. This was all during the Anglo-Spanish War of the late 16th century and early 17th but generally the British involvement in the slave trade represented a way to hurt the Spanish and profit without much thought to the sale of humans in a foreign nation by those engaging in the acts. It wouldn't be for another 20 or so years that English colonial slave labor would start to develop in the Caribbean and American colonies to replace servant labor found there since their founding, and it was during all of this hubbub that Harrison made his comment.