[Shannon]

What kind of mother leaves her 18-month old child with a boyfriend who has a drug habit?

I agree with you but I bet a lot of people, including the mother, probably wouldn't. They'd look at his Methadone habit as "being in treatment" because it's legally dispensed. Legal or not, it's still one mighty drug addiction.

[free me]

The mother was/is in a mental hospital.

Plenty of details here:

http://www.joplinglobe.com/local/x1415465705/Police-Toddler-found-dead-had-been-left-at-home-alone

Note the comments by the grandmother and other relatives.