A worthy but rather simplistic exhortation Marco.
In fact it is very easy to justify slavery, and especially the institutionalised slavery Barry described both on economic grounds and as being of social benefit to the slaves. After all slaves are treated far better than paid workers, because they are property. As Chomsky has pointed out, all the arguments in favour of institutional slavery are now used practically verbatim to justify monetarism, the economic destruction of countries which dare to aspire to collective responsibility as a style of government, etc.
All you need to do is to be sure that your particular belief system is the only correct one, and then of course you have a moral obligation to see that this system is perpetuated and spread. To me it is ironic that at the very time slavery was being abolioshed throughout Europe due largely to the concept of the dignity of man as espoused by Locke, Voltaire, Rousseau, etc. the working classes were being subjected to ever harsher degradation, servitude, working and living conditions by the rise of industrialisation and capitalism; but that was morally acceptable, as they were "free" to starve rather than work.
As Robeson saw clearly, it doesn't really matter what label you put on it, or what 'system' it flourishees under, it is the exploitation of man by man that is the problem.