I'm not really sure why this is tagged [social-welfare] or [welfare economics]. It seems to me that more appropriate tags would be [history] and/or [international economics] (since the question involves international relations), and/or [game theory] (since the question involves putatively rational behavior in a multi-agent context).
Having said that, here is my (uninformed) response. In a case where a much stronger group murders, enslaves and expropriates a much weaker group, there can be a net economic advantage (for the stronger group). The European conquest and colonization of the Americas and their abduction and enslavement of millions of Africans are probably the most obvious examples. Of course, genocide, slavery and mass expropriation do not guarantee long-term economic growth, as can be seen by comparing the economic histories of North America and South America. Furthermore, the influx of easy resources following an easy conquest can create something analogous to a "resource curse" or "Dutch disease". For example, the massive influx of silver from Spain's American conquests initially made Spain "rich", but eventually it led to inflation. It also caused the Spanish economy to stagnate relative to the dynamic economies of northern Europe (Corden, 1984).
However, in a military contest between actors who are roughly evenly matched, I believe the historical record shows that it is always a negative-sum game. Even if there is a clear winner, the wealth which the victor gains from plundering and/or enslaving the vanquished state almost never justifies the amount of blood and treasure that was needed to obtain the victory. So the upshot is that these military conquests cannot be explained in terms of the economic self-interest of the warring states.
But it might be a mistake to treat states as monolithic rational actors. Every state has a governing class, and it may be the case that the war was economically beneficial for the governing class, even if it is economically detrimental for the country as a whole. For example, it may be that the Iraq war was a huge net economic loss for the United States, as argued in the book The Three Trillion Dollar War, by Joseph Stiglitz and Linda Bilmes. But at the same time, it may be that certain individuals and firms associated with people in the Bush Administration (e.g. Haliburton) reaped a large economic benefit. So from the perspective of those people, the war was "economically rational". (Note that I say "it may be....", because I don't have enough information to back up this speculation.)
It is also likely that some wars and mass-murders just have non-economic motives. For example, Hitler was motivated by race-hatred at least as much as by economics. Stalin's purges, massacres, and genocides were motivated by a desire to consolidate his political power, at least as much as by economics. And the Crusades and the Thirty Years War were motivated (at least initially) by religion, not economics. (Certainly, they were a net economic loss for almost everyone involved.)
Afterthought. (added later) Here is another data point. At the beginning of the twentieth century, the economies of Western Europe were closely integrated in a big trading block. Many people thought that a major war between Western European powers was impossible, because it was so obviously against their economic interests. We saw how that turned out....
