I'm not sure what was unclear about the Emperor's intentions.
He fears that mankind will be wiped out since Ains is an undead that has absolutely no use for the living and even threatened to *literally* wipe out the whole world.
What else but unite against such foe. An alliance will only delay it.
On top of that, sweetmouthing Shaltear/Albedo/etc. into thinking that they should rule Nazarick doesn't sound like a bad idea to me, considering how it might lead to an internal struggle within that new kingdom and the fact that 2 of them already acted out of line in less than a few minutes after meating them.
Not to mention that demons tend to be extremely powerhungry in fantasy worlds and this is probably true for that world too. Tough luck though, Ains' minions are obviously extremely loyal.
On the other hand however, they even threatened to fight each other a few times already..