I think this is a good intuition. Particularly in light of the account of MH's parents (via Emmabelle) taking in a younger child when MH was a teen. MH's complaint's of being unwanted and in the way might be related to anger at being supplanted by a younger child who gains love, affection, and attention MH wished to have directed to herself.
As you know the legal definition of insanity has little to do with actual mental soundness. Knowing that she did wrong, planning, and hiding her crime don't exclude the possibility that she was psychotic/delusional about Sandra Cantu. The manic or schixophrenic may create a delusion to account for serious negative emotions being experienced them, and project an explanation for negative feelings onto other persons to account for those feelings.
My theory is that MH was enormously angry, resentful or even jealous of Cantu, and became fixated on the notion that Cantu was a wicked, dangerous influence on her daughter, even demonic in reaction to the resentment and jealousy, because MH is mentally ill. And MH took an opportunity to eliminate the perceived source of MH's bad feelings.
