No, slavery was, not evil, but bad. I'm glad it ended.
Some slave owners did evil things, but the possibility, and i think the majority, of slavery as practiced was done without malice. Evil requires malice, and you can practice slavery without that.
Anyway, that is irrelevant, one day a slave was a valuable piece of property, some people write something on a piece of paper, and then it is illegal and you just lost a lot of valuable property as a slave owner, did not get compensated,
It isn't logic, it is law. Slave owners broke no law prior to 1865.