Why is slavery wrong

Let me ask you this. Would you like to be treated like dirt and have no say in anything. And to get punished for no reason? Let me guess? No. Think of yourself being a slave. Imagine how things would be. That is why its wrong

Because people are not property.

jjwoodhull hits the nail on the head quite thoroughly.

From a philosophical perspective though, before we can conclude that slavery is wrong, we must make some fundamental assumptions. To greatly simplify this from a broad encompassing view, we shall jump to the assumption that each individual person is born with authority over no one but themselves. Any other authority that a person has is authority that others have given them or that they have taken by force. To take authority by force is no better than theft. Slavery can only begin when someone seizes authority over another, thereby stealing that person's authority over themselves.

I shall assume here that theft is wrong. Therefore if theft is wrong, and slavery is theft of one's authority over themselves, then slavery must be wrong. QED.

DrLang puts it very well. I would have started with the fundamental assumption (impossible to prove) that the following is true:

"We hold these truths to be self-evident, all men are created equal, that they are endowed by their Creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness."

If you accept this, then slavery must be wrong, as it deprives men of their equality and unalienable rights. Unless, that is, you believe that slaves are not human. And that has often been the argument put forth by slave holders - that their slaves are less than human and thus can ethically be treated like property, much as we treat cattle or beasts of burden.