Rome was an empire built on slavery and conquest - like all empires. It was therefore an immoral State and in opposition to Natural Law no matter how positively romantic natures interpret Roman culture.
The opposition between slavery and freedom can be found most sensitively in the conflicts that occur between indigenous cultures that have occupied a given land for centuries and imperial cultures who barge in those lands much later on, expropriating (i.e. taking out of their property) the indigenous people from their land in order often to exploit and enslave them (e.g. through forced labour and tax).
Imperial cultures will of course justify, i.e. try to make just, their immoral conquests by claiming they are either 'civilising' (i.e. taming and enslaving) lower cultures, bringing them 'enlightenment' (e.g. by bringing them the phoney 'democratic' system as in the last Iraq invasion) or for phoney reasons of self-defence (again as in the last invasion of Iraq and that country's alleged 'weapons of mass destruction').
We can see therefore that there is nothing new under the sun in terms of the techniques and justifications for imperialism, which is intrinsically and inherently immoral. And expropriation of indigenous people is a wrong because all natural law rights are in essence property rights and all wrongs under natural law are a form of theft of property be it land, body or goods.