Many people argued that slavery was bad for both the slaves and the slaveowners. The idea here was that it made the slaveowners into tyrants.
If you think about it, no one in a democratic society has (or is supposed to have) absolute power over anyone else. We believe that power corrupts and, as they say, absolute power corrupts absolutely. This is what happened (some say) to slaveowners. Basically, owning slaves let these people do what ever they wanted to other people. It encouraged them to think only of themselves and to use other people as tools to get what they wanted. (This is also something we try to teach against on a personal level -- we tell our kids not to be selfish, for example.)
Think, for example, about the impact that slavery seems to have had on the sexual morality of some male slaveowners. They had a captive bunch of women who could not refuse to sleep with them. This corrupted them and made them forget the idea of fidelity to their wives or of trying to control their desires.
Owning slaves, then, corrupted people by letting the forget about controlling themselves and by allowing them to do whatever they wanted with respect to their slaves.