I'm not quite sure how her being "anti-gun" matters in this situation. Long term Mental illness, temporary psychosis, the revenge factor, whatever you want to call this case is not a respecter of political views. People who are "pro gun" and hard core "2nd Amendment activists" have committed these crimes, as well. I don't know how it has any bearing on the larger argument that the causes they agree with have merit, or not.
This woman made this decision, a very horrible one, and took the lives of her children. How she felt about the 2nd amendment seems to be an unimportant consideration.