As I read it I saw the Former as more of a Adonalsium, or another Shard (like Honor) and Decay as Odium. Slaughter and Despair sound much like Ruin and Preservation to me. The way I see it Brandon may not have figured out the full Cosmere by then, and wanted it to put Odium in the same level as Adonalsium. My reasoning is mainly because Mekkal behaves a lot like Preservation when he's about to lose his mind, he doesn't remember who he is and what happened. He feels like a force that has no mind, as if it was Shattered. And Agaris is a lot like Ruin captive. He can contact the people of the world by pulsing and influencing his followers. Lowering Mekkal and Agaris into Slivers or Splinters seems quite low, since they feel a lot like shards. Leaving Decay and the Former in a higher position.