Using Roe v Wade as Case Law

Justice Blackmun wrote the Roe v Wade decision,    
Giving his reasoning, his rationale for it.    
He had the nerve to use "moral judgment" as the basis for summary execution of the fetus.    
I mean, because the fetus lacks "moral judgment" - a prime human trait -    
It obviously is subhuman, a non-person.    
Therefor it can be killed with impunity.    
That sums up Blackmun's reasoning.    
   
Using that same line we could say that killers who are retarded (such that their sense of right and wrong is dim at best)    
Deserve the death penalty obviously.  
because of their limited "moral judgment."
  
I bring this up because the supreme court had ruled against the death penalty for retarded people who commit capital offences.    
The reasoning goes like this.  
Because they are mentally impaired they cant be held to the same standard as the rest of us.  
They  "deserve a break today" and always.  
My point is that this goes against the gist of Roe v Wade !!  
If a court decision is good and right  
It should be usable as "case law."  
If Roe is right why then is it NEVER used as the basis of other court decisions??  
This is a glaring sign that Roe is deficient.  
Even no good, even totally worthless.  
Even harmful to society, morally bankrupt.