Aside from the fact that what she did was reprehensible and unforgivable, maybe some of us are so thrown by this one because of the way it happened.
For me, I think that most of my problem with just pronouncing her evil or crazy comes from the level of organization. She did very organized things to pull this off. She bought the gun, she bought the ammo, she kept her children feeling safe enough that they didn't call for help ahead of time, she separated them in order to attack them, the list goes on.
On the other hand, she did very disorganized things, at the same time. She did not leave the scene, she did not clean herself up, she let a message out to someone close to her that did alert them to trouble, she made no effort to hide the bodies or her actions.
She moved at least her daughter to a comfortable place and covered her. That's a sign of caring, and so damn out of place among everything else here.
That's what's screwing me up. If the entire thing were highly organized, I'd call her evil. If it were completely disorganized, I'd call her crazy, maybe still evil. But this is all over the map, and that's why I can't wrap my head around it.