I am one of those who would not like to think Ember was purposely harmed but I am willing to believe it and would not be surprised. I was leaning towards accidental because if he couldn't take the burden - he could have easily given her back but maybe he wasn't thinking that way or lashed out in anger. When he fled, I started to think in the direction you are going.
There is really no logical or rational thinking with an addict. It could have been an accident, anger, neglect. I don't think we could know without an autopsy, if she is deceased and is found in time for a coroner to find a COD.