@Lucrecia
I have already covered the subject of pain. We've seen other demons get sliced and not even flinch while keeping a neutral expression. In all instances, even in the flashback involving Aura getting slashed by the hero, demons have not reacted to being grievously wounded. Even if they felt pain, they do not use any body language to express their pain and it would make sense to me that showing their pain is a concept that completely eludes them. After all, it makes no sense to show their pain, since monsters don't help each other. Displaying pain would instead cause others to look down on you, as you are essentially saying that you are weak when you are displaying pain. Aura must also understand that Frieren does not have any mercy for demons and Frieren was not even looking at Aura so displaying pain was a completely pointless act.
The only instances of emotion we've seen from demons are related to their pride in their magic. From all I understand, the way I interpret those tears is just frustration; frustration at having her magic be turned against her and at having been deceived on mana capacity.