If you look at resources before and after the war with the same metric, then resources have been expended and lost. If you are using a different metric to measure resources before and after the war, then it is the metric that has changed, not the positive sum generation of resources.

OK, we'll stick to the same metric. Do you agree that human skills and abilities can be resources? Do you agree that the ability to exploit a resources is itself a resource?

Suppose country X has a functioning democracy, court system, banking system, and corporations, while country Y has none of those things. If country X invades country Y and sets up a functioning government and economic system, this could well be positive-sum. Country Y didn't gain any material resources, but it now has a greater ability to exploit its natural resources as well as its human potential. This is a clear positive-sum situation.

Once you start killing people (as is an inherent part of war), then any talk of gains and losses goes out the window unless you attach a specific value to specific human lives before and after and are willing to compare those lives lost with material resources. Since victims usually attach a higher value to their lives than do perpetrators, mutually agreeable values for the gains and losses can not be achieved.

One study put a value of around $1.5 million on a human life. You're right that people will be biased when they try to value their own life, so we should probably disregard any self-assessed value.

Of course we need to be able to value lives and trade them off against other resources; we do it all the time when we make policy or safety decisions.

Compelling people to do something against their will (i.e. slavery) is a negative that can not be “balanced” by what ever positive things the slaves might generate. That is why slavery is wrong, no matter how “productive” the slave masters compel the slaves to be.

Have you read the articles on this site about utilitarianism and deontology? This sounds like a deontological position; I think most of us on this site would disagree. Not about slavery being wrong, but about why it's wrong: that the harm to humans outweighs the benefits.