If the time traveler was willing to kill or enslave hundreds or thousands of persons to prevent the horrors of slavery and the deaths of hundreds of thousands in the Civil War, he could go back to colonial times to prevent the adoption of slavery in the colonies.
Many stories of early America imply that the Pilgrims at Plymouth were the founders of the colonies, even though the Jamestown setters were the very first, a distortion partially caused by dislike of slavery.
So he could go back in time to 1608 Jamestown and wipe out the colony there. But that disaster might prevent the 1620 settlement of Plymouth. So maybe he should wait until after the 1620 settlement of Plymouth and then wipe out the Virginia colony.
Fortunately the Virginia colony was still small in the 1620s. in 1622, just two years after the Plymouth settlement, the Powhatan Confederation launched a surprise attack on the Virginia settlements, killing three or four hundred men, women, and children, about a quarter to a third of the entire colony.
The Powhatans then waited for the English to surrender or to sail back to England. But the settlers instead fought back in a war that lasted for years.
So the time traveler could find a way to make the Powhatan attack stronger.
He could go to Spain, hire some ships, and hire some Spanish mercenaries to attack the English in Virginia at the same time that Powhatans attacked. Since the Spanish government claimed all of North America and considered all English settlers criminals, the Spanish government might not frown on the expedition.
Maybe he could also organize a French and/or Barbary pirate attack on the Jamestown colony. There is nothing like multiple attacks at the same time to make the survivors abandon their colony.
He could go to the Powhatan leaders and urge them to make a stronger attack and to keep it up after the first day. He could advise them to make an alliance with the mighty Iroquois in the north. They could agree to become subordinates to the Iroquois in return for being reinforced by hundreds of Iroquois warriors to make the surprise attack even more devastating. And maybe he could persuade or bribe the Dutch traders at Albany to send one or more ships armed with cannon to smash the palisade at Jamestown and capture it, leaving the English colonists without a secure fortress during the Powhatan attack.
He could provide the Powhatans with many guns and much ammunition.
Thus he might wipe out the Virginia colony and prevent it from developing a slave based economy while keeping the Plymouth colony.
And he would probably have to repeat that over and over again when new colonies in the southern USA are attempted. He should give the southern Indian tribes vaccinations against old world diseases and other medical care - thus saving many thousands of lives even if his plan fails - so they will have more warriors, arm them with guns and ammunition, and whenever a new southern colony was attempted bribe with trade goods a wide coalition of tribes to attack and destroy the colony before it could develop a slave based economy.
Thus he might delay for generations or centuries the settlement of the South and the development of a slavery based economy there, and make the southern population much smaller during the 1860s if there is one at all, and make it easier for slavery to be as peacefully abolished in the entire USA as it was in the northern states.