There was a reason why Eren was so proficient at torturing people.
Reva Rain's demonic ritual required her to torture mortals before killing them. The sudden spikes in the emotion of pain and suffering made her ritual more effective for her, allowing her to progress by leaps and bounds in her elemental attainments.
Reva Rain was also able to strengthen her Ability by torturing mortals. So it was imperative for her that she inflicted more pain, asking her henchmen to do the same.
Torturing mortals was a delicate job.
Mortals' bodies weren't strong enough to sustain the same level of torture as rankers. They could die easily, making the demonic ritual useless or less effective.
The Paladin of Pain was taking huge risks by distributing defective ranking techniques among mortals. That's why she didn't like it when mortals died without allowing her to extract their worth from them.
