Why were slaves treated badly?

Because they could be treated badly. They were seen as sub-human beings. They were considered to be property, like cattle or horses. Thus, it was perfectly legal and acceptable to beat, abuse and torture them. George Washington was a slave owner.

Some people are violent and sadistic by nature, and they only resist abusing others if they believe they could get into serious trouble for it. They might fantasize about beating up their boss, but they don't do it, because they would get fired. They might want to murder their ex-wife's new husband, but they don't, because they're afraid of going to prison. But if they know they can get away with abusing and mistreating others, they will.

Most people are not so violent and sadistic that the only thing that stops them from hurting people is the fear of ruining their career or going to prison. However, most members of the human race have the capacity and the latent desire to abuse and mistreat other living creatures. Most people, under the right conditions, can become quite sadistic and cruel.

All humans have a sadistic side, some more than others. The majority of people can become brutal and sadistic if society is plunged into chaos, or if they are drunk with power. A small minority of people enjoy hurting others, and will only refrain from doing so if they think there will be consequences. And another small minority of people are truly decent and kind-hearted, but can become violent if they are provoked enough.

Most people will become corrupted by power, if they are given too much of it. Too much power also means that you can get away with behaving badly. It means there are no consequences for your actions. And once you take away the consequences, people become dangerous, especially if they are sadistic by nature to begin with. Powerful people treat less powerful people badly simply because they can.

Too much power can corrupt anyone. But for some people, even a little taste of power -- like being promoted to Assistant Manager at a retail store -- turns them into tyrannical despots. Usually, people who can't handle even a little taste of power are those who have always felt weak and impotent, so the little bit of power goes to their head because they have always been so hungry for it.

Your evil boss might be mean to you, and harass you, and make false complaints about you, and do anything else she can to get you in trouble, because she doesn't like you and has the power to hurt you. But she doesn't behave that way around her own bosses. She doesn't have any power over them, so when she is around them, she is meek and humble and deferent.

But, while your evil boss may be able to get away with harassing you and abusing you to a certain extent, there is a limit to what she can do to you. If she goes too far and abuses you too much, and if you have proof that she is harassing and abusing you, you can complain to the company and have her fired (or at least, have her disciplined and forced to stop abusing you).

But slave owners had absolute power over their slaves, and, as a wise man once said, "absolute power corrupts absolutely." Your boss doesn't own you. Slave owners owned their slaves. Thus, slaves had no way of getting their owners to be held accountable for the abuse they inflicted. There was no one to complain to. There was no one who could -- or would -- discipline a slave owner for treating his slaves badly. Slaves were property, which meant a slave owner could do whatever he wanted to them.

Slave owners treated their slaves badly because humans have a cruel, sadistic side that, for some people, is only held in check by the knowledge that there will be consequences for their actions. Some people will gladly take every chance they get to abuse others, if they know they can get away with it. And even normal, non-violent people can become sadistic if you take away the consequences.