While I would agree that believing that slavery is "good" would imply poor morals, I also don't think that morality is objective, and while I think my criteria for evaluation of it are fairly good, I don't think that I would be comfortable making that level of value judgement over someone holding an opinion, particularly if the held that opinion out of ignorance or unknowingly incorrect things they held internally as being axiomatic.

I agree in principle about things that, in your words "divide for the purpose of uniting" to do more harm then good. From the standpoint of idealism, I think that the best way to go about ending inequality is to simply cease treating people unequally. We can't just go straight from here to there though, and I don't have good solutions. I suspect that it's the type of thing that phases itself out as cultural patterns live and die, and that's slow. The ramifications of that are a separate, stupidly large can-of-worms topic.

And people have the right to be offended or not offended.

I agree that people "have the right" to be offended or not offended. I mean, in case the above is indicative of me not expressing myself clearly, that people do not have the right to suppress other people's expression of their opinions because of their offense. And people are not obligated to be sorry for offending somebody by having and expressing an opinion. And people do not have the right to expect that no one will be offended by their opinion. And people do not have the right to only have opinions expressed to them which they do not find offensive, assuming they are not in a ridiculously coercive situation, in which case the breach of right by the gun being held by their temple is probably of bigger concern than the offensive opinions being expressed to them forcibly. Outside of the context of extremist opinions, which are the majority of examples that get brought up in this type of discussion, there are way too many people for that to even be a feasible scenario.

A morally corrupt person has the right to hang out in the Hall of Evil if they would like.

As long as everyone in the "Hall of Evil" is consenting to hang out there, I don't think I could make a moral judgement about them. I could make one about myself, and how I would not feel morally "right" if I was doing what they were doing, but if they know what they're getting into and they're consenting to it, I could not call it morally corrupt. I know enough people that are legitimately into things that make me uncomfortable to think about participating in to not be able to hold another opinion while being intellectually honest with myself.

But the purpose of the original post being discussed is that there is a way to apologize, and for that matter a reason to apologize. I personally think the apology was warranted.

I agree. And I think the second apology they posted was much better. An apology here is absolutely warranted because they ended up making a whole bunch of people think that they were ignorant, or bigoted, or malicious in someway when they are (most likely) not, and did not intend to be so ridiculously insensitive to stuff they knew about. They made a stupid mistake, caused a bunch of people to be offended where they did not intend to, they're in the process of owning up for it, and that's good and will likely cause people to respect them a bit more.

If they posted that they thought "women should stay in the kitchen", and they really thought that, then the most honest apology they could make would be that they were sorry people got offended. I wouldn't agree with them, I would actively show people reasons why their opinion should be ignored if they were campaigning for laws requiring "women to stay in the kitchen". If they realized later they had been publicly holding a pretty ridiculously offensive opinion for no good reason, I would expect they would be apologizing, probably for their ignorance, but mostly for actions and offending people. If they just held it, honestly thought it, expressed it, and offended somebody, I see no reason for apology, and don't see how any genuine apology, other than "being sorry you're offended" would be possible.

Do I think slavery is defensible as a moral opinion? No. I think that if someone does, they should be able to express that, and that if they hold that opinion out of ignorance or misunderstanding that that ignorance or misunderstanding can be addressed. I think that suppressing the expression, the ability to express an opinion, any opinion, because it's offensive or "morally unsound" is dangerous. I don't think that many, if any, of the "extremist" opinions I've heard have enough weight that they should be considered so dangerous as to be banned. Doing so does nothing to solve the problem of people holding those opinions when they do not know they should not be.

Slavery, by the way, is alive and well. A number of people not only think it should exist, but actively purchase humans. A large part of minimizing or stopping that is making it so that when people who think that slavery "is just how it is" -- because they've been surrounded by it, or had ideas drilled into their heads as children, or grasped onto some fallacy they didn't identify -- are capable of being presented with information to the contrary in such a manner as to actually change their minds. This does happen. It does not happen when expression of opinion is suppressed.