As a civ, pursuing good will likely lead to more death in the future.
For example, let's say in the next few turns, the Trelli decide to join with us due to economic/military troubles on their part. In a few turns after that, they even become willing to give up the slave trade because they converted to our Crow religion.
Problem is, it turns out that if we do cut off the slave trade, multiple major nations will outright crumble as they lose their workforce, and even more will crumble as those nations collapse.
The 'good' option, at first, seems to be to just outlaw the slave trade. Others will be harmed, but it is the right thing to do, and those nations were built on the backs of their slaves anyways. But even ignoring the hundreds of thousands who will suffer because of that decision, it can also harm us as well. As those nations collapse, we might see an upsurge in raiders. Greenshore is sacked by a marauding band from the collapsed tin tribes, Trelli is beset by pirates, and so forth.
All that death and destruction, caused by pursuing 'good'.
Nations cannot base their agenda's on morality, they have to base it on a combination of their own self-interest and the desire for stability of the world. Morality can affect these decisions, but they can never be the basis on which the decisions are made.