Slavery based on kidnapping is wrong because kidnapping itself is wrong. Someone who kidnaps should be, upon conviction, executed.
However, slavery as a form of punishment for crime or as a deterrent for one's enemies is just. In such cases where restitution is the punishment, but the criminal cannot afford to pay their debt, then they would be put into indentured servitude for 6 years, or until the debt is paid off, whichever comes first (separate from the victim, of course), and all the proceeds of his labor go to pay off the victim of his crime. On the seventh year, he is released and any remaining debt is wiped.
That is what the Bible says. It also says for slaves to honor their masters, and for masters to not mistreat their slaves (talking about slavery as a deterrent for war).
Begging the question is a logical fallacy. You should avoid using it.
Thankfully, appeal to popularity is also a logical fallacy. You should avoid using it as well.