I was thinking of something I had read some time ago that discussed the status of slaves in the different legal systems. How it worked out in practice would depend on various things—being a slave on one of the sugar islands often meant a short life expectancy because the slaves were worked so hard (since replacements were cheap). In the English colonies it probably made a lot of difference if you were a household servant in New England or a plantation field hand in Virginia or South Carolina. And the white indentured servants didn’t have a good time of it either—many of them died before their period of service was up.
I think you can make a case that the reduced legal status of slaves in English vs. Spanish colonies was an unfortunate side effect of the generally more egalitarian English society.
With very rare exceptions, almost none of which were applicable in the colonies, all English freemen were equal under the law. (Actual practice, as in all societies, was somewhat different.)
In Spanish colonies, OTOH, there were massive differences in the legal status of various groups. Notably, if I remember correctly, government officials, military officers and clergy all had their own separate legal systems, not being subject to the general judicial system. In practice this meant these groups were essentially above the law, as “their own kind” was highly resistant to punishing them.
In English colonies you thus had the majority of society in one group. While it hadn’t been verbalized yet, the basic fact was that “all men are created equal.” The only groups outside this were the black slaves and the white convicts and indentured servants. So you have a big group that are “people,” and a smaller group far below them separated by an impassable gulf that are “not people.”
In Spanish colonies nobody was equal. There was a spectrum of status and rights from the slave to the Viceroy, with very slight differences between any two adjacent individual statuses (statii?).
So the Spanish had no difficulty thinking of slaves as people, they were just people with a different and lower status.
The English and later Americans had considerable difficulty doing so. All people are equal, but the slaves aren’t equal, therefore they must not really be people in some sense. This notion reached its highest (lowest?) point with the Dred Scott decision.
Interestingly, the English colonies in the New World dropped common law when it came to slaves and imported the foreign Roman civil law definition of slaves as “speaking animals,” with no rights whatsoever under the law. This was softened to varying degrees in different areas by the influence of Christianity.