To wonder if people who are objectively bad people know they’re bad, or if they’re just delusional?

No, either they don’t reflect at all or they believe that even the dubious things they have done are justifiable or understandable and that everyone would do them if they had the change

Yes, I had an ex like this, he would have moments of lucidity, and tell me I should keep away from him, because he was a really bad person.It was a bit sad really he tried to be better sometimes, but couldn't keep to it.
Eventually I listened.

People have different values. I notice frequently in life and on Mumsnet that some things I might regard as selfish or greedy or unkind, some people think of as entrepreneurial or not being a pushover.

You can't call somebody who lies or manipulates an objectively bad person, there are plenty of instances where you could argue the opposite.
An ex who cheated - sure, they could be a terrible person in your eyes, and probably the eyes of many others, but you can't say that they are objectively bad. What's going on in the specific situation?
If somebody does something that is considered to be terrible, it's hard to say if they're aware or to what extent. They might be unaware and never/later register their behaviour. Or they may be aware, but simply don't give a shit or are unwilling/unable to care for whatever reason.

I don’t think I’ve known any really evil people, but I’ve known/currently know some amazingly selfish ones! They don’t consider other people, such as their family members, at all. It’s quite extraordinary sometimes how blind they are to how much pressure and stress that they put on others.

I have experience of a horror of a person like this. You would be forgiven for thinking he’s some sort of demon wearing a human suit, that’s how bad he is, yet superficially very charming and passes as normal.

I think he knows what he does is considered wrong by normal people because he masks it and lies about it.

But I also think that he doesn’t believe normal morals and ethics apply to him.

His ethical code seems to be -
What he wants / likes = good.
What he doesn’t want / like = bad.

I've pondered this all my life. There were some truly nasty, sneering bitches in my year at school. Did they know they were nasty? Surely it's not possible to be that big a cunt and not be aware?

I think everyone thinks they have the right motivations. It’s just that some people are motivated by the wrong things.

I’ve known a few people with compulsions that make them lie or do strange stuff. In the moment they really think they are making the correct choices.

I hope this isn't too lighthearted. I think it has a serious point to make.

Some people (psychopaths) have no concept of right and wrong in terms of hurting other people because they view other people as objects. Psychopaths aren't just serial killers, many are very successful. Lots are surgeons, CEOs etc.

Other people actually get a thrill out of hurting other people because they are mentally ill or very damaged or both. They usually don't admit this to themselves on a conscious level though and tend to fall into the category that will always try to justify their own behaviour and gaslight others to blame them instead because convincing themselves they are in the right and it's all someone else's fault is the only way they can reconcile the cognitive dissonance.

I think there are probably very few people who know they are an objectively bad person and consciously recognise this and admit it to themselves. Nobody wants to feel bad about themselves so if they did, and did not fit into either category above, the realisation would be sufficient to make them want to change (even if just for the selfish reason of protecting their own ego).

My ex would occasionally admit he knew he was ‘bad,’ although it came along with a lot of self pity.

IME, those who do "bad things" tend to be able to see themselves as the "good guy" or in most cases the victims and everyone else is wrong.
Those that are sly and get others to do things, tend to think that its not them that have done it so they can't be bad.

There’s a great video on YouTube on the differences between narcissists sociopaths and psychopaths.

I'm often astonished at the way the most inocuous act can be reinterpreted by someone with an agenda.

Some people are deluded. I know of people who abused their children but go to church and help others.

Some people enjoy harming others, they see others as weak and easily manipulated. Society often victim blames, asking why the victim put themselves into a position of harm.

I believe that everyone is capable of evil given the opportunity.

I don’t think anyone thinks of themselves as a villain. I think some people do bad things and convince themselves they are doing it for the “right” reason. I think narcissists just don’t care and will do whatever they think gives them an advantage and are basically amoral.