Do you realize this is completely evil? It is forcing a choice to do evil. As someone who has been subjected to SRA/MK from birth, this would be a situation I and many from this background have been forced into. The other "choice" would be refusing to kill, but being forced to do it anyway.If my choice is to either kill a small child or else torture and then kill the same child, I would choose the mercy killing.
That is provided that those are the only two choices and there is not a third (such as a justice killing of the one requiring this of you).
If by choosing nothing at all, the default is that the child will be tortured and then killed, then by choosing nothing you have chosen for the child to be tortured before it dies.
My point is, this "choosing the lesser evil" is straight from the occult. In truth, you are not responsible for what someone else does.