She is not one of their dead, the positive dead. Though killed by her son (in her sleep), in death she is with her son, the negative dead.

As this heartbroken town tries to process Friday’s horror, there is considerable anger toward Lanza’s mother. Her name is noticeably absent from many of the impromptu shrines, memorials and condolence notes placed around town.
You can read the whole story and see the ghost house of the killer and his mother here. All I see is isolation.
“Why would a woman who had a son like this, who clearly had serious issues, keep assault rifles in the house and teach him how to shoot them?” she said. “To deal with that, there’s a feeling here that we’re just going to focus on the 26 innocent people who died at the school.”