For Kai, the man was lying on the surface was the one who called Monster. How he could sacrifice other's life? It was acceptable if Sima to offer his life to save them, but it was not the case. Kain forced someone to do it for his own sake.
Moreover, Kain's consciousness left him due to the high physical strength he received. So Kai wrapped him in a post, as well as the help of Isabella's binding magic.
"We have to leave, Isabella. We should not kill this man right away, and let him the severe pain he had never think in his entire life."
Kai pushed himself into a stood-up position and seemed ready to leave this place. For him, this man did not deserve painless death, or to be exact, he was going to torture him until he wished to die.
Despite the fact that Isabella would like to stay in this place, still, he can't see how desperate Kai was. So in the end, she just simply nodded her head.