Slavery was ended *because* of much bitter bloodshed. When the war began, there was no intentions at all of freeing the slaves. It was nearly two years later that Lincoln decided to issue the Emancipation Proclamation, which only freed some slaves, and was seen as a war tactic to weaken the South.
Once the war was over, Lincoln pushed for the 13th amendment because if the South was allowed to keep their slaves, their economic engine would have once more produced massive amounts of Foreign Trade, but this time the South would be intent on using that money to get revenge for the bloodshed perpetrated upon them.
They would have minimized their investments in Northern Shipping or New York businesses such as banking, insurance, warehousing, and they would have insisted on using Southern companies for all their transactions. Additionally, there would never be another Southern vote in the Congress for anything wanted by the Republicans. The entire block would be forever voting against anything desired by the power block which won the war.
By freeing the slaves, not only did the Washington/New York power axis dissolve most of the capital held by Southern interests, they also prevented them from reestablishing their former profits from foreign trade. Additionally, by giving these freed slaves the right to vote, they diluted the political power of these Southerners who would be otherwise expected to do everything of which they could think to thwart the political desires of the Northern power block.
Freeing the slaves solved a lot of potential political and economic problems for the Northern Interests who needed to stop the South from gaining economic independence from their control.
But don't think anyone did it because it was the moral thing to do. This was a calculated move to retain power, and nothing else.
Also, technology was going to make slavery obsolete anyway.