I think that might have been touched on in the write up but then that's just me.
If he is a monster, he should live to stand trial so, either way, saving his life makes sense.
As to his monster sister, I think I am being generous in calling her a sociopath, heartless f@#$ing bitch is more like it. Remember, she initially agreed to donate so he couldn't be that bad.
Assuming you ever get him to trial, and how many innocent lives are lost in the waiting?
Save one at the cost of two, or save two at the cost of one?
A sociopath would have never agreed to donate in the first place and would have been unmoved by pleas, manipulation, need or any other social imperative, because they are, by definition, asocial.