"A good study out of New Zealand interviewed six moms who killed their children, presumably for altruistic or psychotic reasons and all said that being a good mother was important to them and bitterly regretted that the child had died when they were well," she said. "If they come back to their senses and sanity after treatment, it will be horribly traumatic for them, as well."
"I think they love their children, but many are mentally ill at the time," she said.
Police will never know if that is the case with Armstrong, who plunged her car into the water just 10 minutes after one of her relatives called 911 after hearing an argument over the phone at Armstrong's house.
Armstrong took her four children -- ages 10, 5, 2 and 11 months -- and drove the car off a boat ramp about 7:50 p.m. in Newburgh, N.Y., a riverside town about 60 miles north of New York City.
"She was a very good mom," neighbor Tina Claybourne told the Associated Press. "She took care of her kids. She always was with her kids."
Investigators call child-killers like Armstrong "family annihilators," for the way in which they take their own lives and their children, too, said Ken Lanning, who is retired from the FBI's science behavioral unit.
In a famous New Jersey case, John List, a rigid religious man, lined up his entire family and shot them one by one. "But he forgot to kill himself or couldn't do it," said Lanning.
"I don't know the facts of the case, but this woman apparently decided to commit suicide and that could have been the result of any number of things -- clinical depression or the reality or perception that life was not worth living," he said. "The second issue is what does she do with her family? If she has young children, she has to decide what to do with them."
The fact that Armstrong had an infant suggests she may have had post-partum depression, according to Lanning.
"The point here is that it's a sad thing when these people kill themselves, but why take three or four kids with you?" he said. "Some mothers think, there is no one else to care for them and we will go to heaven together, if they strongly believe in they will be reunited in the hereafter. But it doesn't justify it or alleviate the fact that three innocent kids died."