Yes, slavery is very evil. Thank you for pointing that out to us. Unfortunately for your argument, it happened to be Legal under Union rule in 1861.
Please take this into account in your future attempts to once again remind us how evil is slavery. That you feel the need to keep repeating the obvious makes you come across as a bit simple.
And it probably would have remained in that sorry state for some time if the hotheads in the South hadn't rushed to war upon Lincoln's election.
But no, they did the one thing that would assure maximum destruction, and a speedy end to the infernal institution of chattel slavery.
Oh well. "The Almighty has His own purposes."