Remember the translation of Final Fantasy Tactics leaves much to be desired. It was probably a missed translation, there are so many grammatical and spelling errors in it that it boggles the mind.

Also, "he's the leader of Death Corps" is not grammatically incorrect, even though it does sound strange. If phrased that way, it speaks more as if Death Corps were a company, or an organization (which they are), just like we might say "Ken Lay is (or was) the CEO of Enron".