Liberals tend to overlook economic implications of all public policy issues. Owing a slave had an implicit cost. Most employees in the period at the bottom end of the agricultural economy were earning only subsistence wages--a slave included not only the subsistence but also other costs related to security and management. Those costs did not make the institution uneconomic until the cotton gin--however at the point use of the gin because widespread, the cotton farmer with slaves made less money, or lost money, to the farmer who did not have slaves. That spelled clear ultimate end to the system at a finite point in future history.
The death of over 400,000 Americans was simply not necessary to enforce the ultimate end of slavery. The only slavery issue was timing.
Out of 20 or more slave-holding countries, Haiti and the United States resulted in mass violence/war.
The cost of the WBTS was $6.6 Billion (1860) - that dollar amount is said to be enough for the North to buy freedom for every slave in these United States and give them 40 acres and a mule. Of course the other cost was human life - estimates say one dead soldier to every six free slaves.
Lincoln was foolish for going war - the costs in terms of human life, individual and states rights, money, and destruction were harmful to the Union he fought so hard to preserve.