Kain wasn't having Darius suffer just because he was a sadist.
The first time this procedure was done on Ferrin, Bea completely dulled his senses and so he felt no pain.
Since Bea wasn't recovered for the director's and Gabriel's procedures, spiritual plants were used instead, but they weren't able to completely eliminate the sensations of pain.
However, Kain noted that the time both the director and Gabriel's soul fragments were able to persist in Pangea, and the quality of their contracts, were improved.
Gabriel's circumstances were unique due to the artificial core that had fused with him and made him have a much more dense and concentrated spiritual power. However, the director had no such boost. Moreover, due to his age, Kain had actually expected for the director to have a more difficult time than Ferrin with activating the array and have a weaker soul fragment. But that wasn't the case.
