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.