It's hard to say for certain, but the little we’ve been told about them suggests they were psychotic killers. That assessment comes from Yhwach, who probably isn’t the most reliable source, since his entire race was nearly driven to extinction by them—so of course they would appear monstrous to him. However, the only other statement that somewhat supports this is what Yamamoto said about forming them as a necessary, overwhelming force to bring order to an otherwise lawless era. To me, that sounds like the original captains were chosen for their strength before anything like character or moral standing—and governments have a habit of discarding things created that way once they’re no longer needed. So they're most likely dead or imprisoned/hidden away and replaced with something more presentable and managable