Freedom*

Luna couldn't kill these Archdemons because they are immortal, but she was pretty sure if she could get Luxen and Ravin through the portal, she could imprison them in Hell forever. 

She knew that it was way better than wreaking havoc in the Vampire Realm.

They still are connected with Luna, but their life forces aren't attached in a similar way to the Vampire blood bond.

The blood bond would make the other mate feel sudden emotions, especially shock, pain, and suffering, if their connection wasn't that strong and unmanageable yet.

Blood Bond is more effective to vampire mates because, of course, they drink blood.

Archdemons can probably mate by blood with whomever they chose, but is bound to the said female is another story.

They were pleasure-seeking rogues, and they do whatever they want.

They weren't capable of loyalty. 

They kill without regard—innocent or not— and they have zero morals.