You first have to define the opposing party. Are you speaking of the slaves or of the other humans who oppose slavery? Because remember, part of the justification is that slaves and unborn babies are not fully human, and thus cannot be the opposing party.
You're assuming it's like killing. By that you're making a moral judgement. Significant part of the population disagrees with you. That means your judgement is not universal and shouldn't be forced on others.
There are moral principles vast majority of the population agrees with but abortion=killing is not one of them. Don't force others to live according to your religion or moral conviction.
