ITA. Everything we know so far indicates that this (especially the murder) was not premeditated. Aside from the gas purchase and ABM withdrawal, most obvious is the need to purchase the garbage bags and hammer. One would be hard-pressed to find a home that didn't have at least one of these items. Why not bring them from home? Neither of the accused had a lot of money to waste.
Actually, I'm confounded by the necessity for a hammer at all. IF MTR did the killing, a tiny 8 year-old would be no match for a grown man's hands, KWIM? (I apologize for the mental image!) There was no need for a weapon or the ensuing blood.
JMO