Let's play devil's advocate here. Let's think about this assuming we don't care about the mass suffering, slavery and murder of humans, which is kinda bad enough already for us to try to end this practice any way we can. Say we are just bare naked capitalists, only interested in profit, past the point even Adam Smith would find horrific.
This is still bad enough for us to care.
We can't use slavery to produce our products because of laws and non-corruption in our countries, nor can we change our system to allow slavery. It would cost too much. So there is no way we can compete with Malaysia who is allow things, official or not. They are gaining an 'unfair' advantage by resorting to this practice that only they can use.
Therefore, even if you are an inhuman psychopathic capitalist (or at least a long-term high functioning one), you should care about abolishing slavery, since it grants those who do an unfair advantage.