Questions About the Mother - Martin Relationship
Did she know that Martin was periodically returning to the room?
Was Martin locked up down there after his brother died & was Martin responsible for the twin's death?
If Martin was so outraged at his mother, why wait for a chance encounter at the hospital to kill her? After all, all his previous murders made some sense and most were well planned. It would have made more sense for him to not kill her, after all he did want her approval & he wasn't just some sadistic killer. Since he was periodically returing to her house, he had ample opportunity to kill her.