I honestly don't think anyone intentionally killed Haleigh, thus no motive applies. I do think that she was harmed by something done to her to get her to go to sleep or as a punishment, i.e. I don't believe she accidently got into the drugs. Drugs=money to dealers and users and they keep them well-hidden, especially this bunch when they apparently have people coming in and out on a regular basis who might be tempted to help themselves. I don't think they locked their stash up to keep the children safe, in other words.
I don't know if Ron caused the harm, obviously, none of us do, but if he did, I think he did not know she was dead when he left but was worried about whatever had happened prior to him heading to werk, this the phone calls. If he had hit her, say, then he probably had cooled down and wanted to make sure she was ok and when Misty wasn't answering, kep calling in a panic. The stop on the way home makes me think he found out something bad during the course of the evening, or suspected it, and decided to create an alibi.
I don't think LE is going to need to worry about a motive in this case; when children are killed, there is never a good reason so LE does not need to try to convince a jury of that.