So, I'm gonna go out on a limb and say that the reason Be'Lakor is over the other chaos warrior's body is because he's going to become the AoS answer to Malal god of contradiction. This would also introduce an interesting wild card, faction/character that could make chaos less of a monolithic faction that would often go after other chaos factions as hard as order factions
Also, hi long-time lurker, first time poster.