Why is Mrs. De Ropp to blame for her own death in Srendno Vashtar?I see everything so subjective, that is, the boy only "desires" she is dead...

Of course, Mrs. De Ropp does not actually deserve to die. 
Her offenses against Conradin are grave, but they are not capital crimes.  However, she
is to blame for her own death because she makes a concerted effort to make Conradin
unhappy.

We are told that Mrs. De Ropp dislikes Conradin
but won't admit it to herself.  We see her doing things specifically to hurt him.  She
sold his hen just to hurt him.  She saw that he had the hutch and decided to get rid of
whatever was in it.  She is doing these things just because she likes to make Conradin
unhappy (though she tells herself she is training him).

So
this is a woman who is really being quite cruel to a child that she is raising.  She
doesn't deserve to die, but she does bring her death on
herself.