Some may feel that it would be cruel of them to bring a someone into the world who faces massive disadvantages.   These same people might feel that it is not necessarily cruelty for others to do so... others who may hold a different set of beliefs.  In fact, these sets of beliefs might be viewed as spiritual (or even religious.)  So, let me ask you, when one set of *reasonable* beliefs construe an act as merciful, why should others shame those who hold that belief set as apologists for murder... or call them eugenicists?  At that point, it seems like a case of "My God is bigger than your God."  The same goes for assisted suicides for the terminally ill.  Some might consider it murder, where others would call it merciful.  

The bottom line is... where two sets of reasonable beliefs clash, it is unjust to create laws that affirm one set while making actions taken upon the other set illegal.