The Spanish priests were trying to show compassion for the suffering indigenous people, who when enslaved died rather quickly, sometimes from disease, sometimes from mass suicides. So they imported Africans, who were believed to be more hardy. Later, British privateers and later privates captured some of these slaves together with the other booty, and in 1619 auctioned off indentured servant contracts in Jamestown for those negroes, and much later the limited contracts turned into lifelong enslavement, only of negroes.
They came with benefits, too. Their music has changed the world, and the writings of Frederic Douglas, Harriet Jacobs, Booker T. Washington, Thomas Sowell and Langston Hughes, for example, have had a huge impact.
We could argue/discuss the positives and negatives all day long, I guess. But from my perspective this country would have been far better off without the introduction of slavery.