Your take is what, mental illness sufficient to cause someone to kill their kids is possible, and when it occurs it...is always also powerful and far-reaching enough to always degrade the ability to accomplish it? And/or being able to plan and accomplish it means they couldn't have been fucked up heavily enough for mental illness to cause it?

I understand feeling strongly about the case, but that position is shallow and doesn't add up. I've seen people undergoing psychotic breaks and heard about many more than I've seen. They can obviously plan and accomplish things, that's kinda the whole problem for everyone else, where their internal wtf becomes external. And unlike merely drug-induced psychotic states, it doesn't just fade with the dose, it dwells and lingers and festers and can become inescapable without help.

The things folks in that state can accomplish make no sense outside of their head, and shouldn't be accomplished, but they don't magically turn into a mush that can't keep an idea coherent and "pull it off". (Bad) Ideas remaining too coherent and consistent is kinda the whole problem? Your insistence of otherwise kinda reveals your lack of experience with the whole shebang, just being honest.