Those 300,000 lives
A good case can be made that they could have been saved.
Slavery was a dying institution and would have probably disappeared in a decade or two. So the question becomes, was it worth the slaughter to end slavery earlier and was slavery really the reason for the war.
Many southern historians say that the war was all about State's rights and slavery was just a peripheral issue. The horror of slavery certainly provides much needed justification for the one of bloodiest conflicts in history up to that time.