Sure, if you look at it from a utilitarian perspective I suppose.

"One's rights end where another's begin" - Morally speaking I agree with this, and I've heard this phrase used by animal rights activists to argue that humans shouldn't have the right to violate animals' (moral) rights to be free, to not be killed, harmed, exploited etc. at least by humans who are moral agents & don't need to do so.

Again, there is a difference between moral and legal rights. Just like in the case of human slavery where some humans technically had the legal right to enslave other humans - and I would agree that those laws were unethical to begin with since the moral rights of those slave owners to do things ("positive" rights) ended where the moral rights of the victims to be free from oppression/harm/etc ("negative" rights) began - many people argue that the current legal rights of humans to, basically, enslave & kill non-human animals, are similarly built on unethical laws, and don't translate to moral rights, in the sense that humans' rights also end where other animals' rights begin, morally speaking (such a position would of course entail action to liberate non-human animals via boycotting of animal exploitation (veganism) as a moral obligation, similarly to how when the laws that enabled people to own slaves were in place, boycotting the slave trade and being an abolitionist would also be considered a moral obligation by most people today).

I would also add that it seems that rights are a human concept/social construct, even just in the sense that we're interpreting what we believe to be ethical/right/moral, even if it's objectively correct; or we're enforcing laws based on what people believe is correct, or in some cases what serves certain people personally at the expense of what most people believe is right if the laws are corrupt/undemocratic.

So I think if we're going to claim that a certain right "just is", since we're the ones creating these concepts even if it's based on our observation of the world and an interpretation that was theoretically objectively correct if not a belief, it falls on us to rationalise and describe how we're coming to these conclusions and what we're basing this assertion of a certain right on. Otherwise, "it's a human right because it's a human right" is just circular reasoning and has no explanation. How are we formulating our basis for what is a human right? Is it legality? Is it moral beliefs or what we reason (or even logically prove somehow) is objectively morally right? Or ... what?

For example, in the case of animal rights theory, many people believe that there are moral rights that animals hold as moral patients, i.e. "negative rights" (= freedom from something being done to an individual) not to be exploited and killed by humans (moral agents), which extend logically from the belief (or fact) of human rights also being morally correct. And in this view, humans by way of our laws, do hold legally the "positive rights" (= freedom of an individual to do something) to exploit and kill animals, but these legal rights are simultaneously violating the moral rights of the animals to not have these things done to them by humans/moral agents.

In this case too, similar to what you said about the human condition, we could argue that something about the condition of animals (which could for example be sentience/consciousness, which they share with humans who are also animals), is the basis for them having these rights, but even then we're still speculating based on what we believe is either subjectively or objectively moral (since in that case obviously what's legal is in contradiction with what's deemed to be moral), and I'm not sure what third definition of rights could be being applied there whether it be in the context of human rights or animal rights.

Human rights describes the individuals that the rights pertain to, no? So those human rights could either be based in legality or in morality, which wouldn't always align. People may also have different beliefs about which human rights are morally justified and which ones aren't. If there's a third kind of human right that isn't based on what's legal or what's believed to be (or, fundamentally is) moral, then what's it based in?

Inherent to the human condition is interesting, but isn't that still a moral stance/belief? Even if you argue that it's objectively moral (and if you don't believe in moral subjectivism/moral relativism) or objectively the right thing for humans to have rights based on the kind of beings that they are, how is that separate from morality? As far as I know when someone says "this is a human right" they're usually asserting that they believe it's morally correct for humans to have a certain right, and that it would be wrong to violate that right. Occasionally someone says "this is a legally protected human right" to emphasise that it's a legal right enforced by law. I'm not sure by what metric rights could be ascribed or theorised conceptually to apply to certain individuals, if not law or ethics.

For example, you could say that the law did violate the enslaved's moral human rights, by assigning other humans a legal right to own them, which many at the time would have also believed was their moral right, even if we don't agree with that today or assert as being objectively immoral. If their human right to not be enslaved wasn't legal or moral, I don't see what the third option could be.

I'm fairly sure human rights can be used to describe either moral rights or legal rights. In most contexts people are using human rights in a moral sense, but it can be used in a legal sense too. If you're arguing for a third definition of human rights which isn't based in morality (what's good) or legality (what's been passed as law), then what is it based in?

They weren't a white supremacist and they were in favor of banning slavery while simultaneously believing it to be an authoritarian decision. They were using this to argue that authoritarianism can be justified sometimes. Your comment assumes that saying something is authoritarian means that you're against it.

But authority can be used/imposed to take away some else's authority, can't it? Or can authority only be used to do something to someone, not to prevent someone from doing something?

If it was legal for certain people to slap certain other people, then the people doing the slapping would have the authority over the people being slapped to slap them. But then if the law was changed and took away their authority to slap them, that would be using authority over those slappers to stop them. Does this make sense? Both can be true at the same time

Do you agree that someone can theoretically have a legal right to do something bad (as in, be legally allowed to do it) without that being a good or moral right for them to have?

I think you're only believing "right" to mean one thing and one thing only, when I'm using it in a sense where legality and morality don't necessarily coincide (even if they do in other contexts, conditionally).

So when I say they had the legal right to own slaves, and that right was taken away from them, that isn't a matter of opinion/belief because that's factually what happened, but that doesn't mean that I think they had the right morally speaking, which is a different concept.

I hope this makes sense.

Legal rights vs moral rights, that's the confusion.

They legally had that right at the time. I don't think they should have had that right, or that they morally have that right. I think we're talking about 2 different meanings of the term "right". In one sense (legally), they had the right, as in it was codified into law. That's not a belief as much as a fact. The part which concerns my belief is whether I think they should have had the right or if they have the moral right, which I don't. I hope that makes sense.