Patriarchy used to mean men were allowed to kill the children the women in their household gave birth to. (No one knew who actually was the father back then)
Presumably that's why it shocks some people less. The notion that a man owns his wife and children is far from extinct.
People are also much less shocked if a woman is raped by her own husband, who is supposed to love her, than by a stranger.
I did get the impression, however, that cases where a mother kills her child or children are very much covered by media. People do consider that unnatural and dwell on it a great deal. Can remember a lot of cases from the top of my head, where the name and location of the murderer was mentioned; while men who kill a woman and her children out of spite because she divorced them, do not often get more than a small newspaper article - it is common, and therefore not newsworthy, I guess.