I assume the child did not have volition, will, intention when playing(?) with the rifle, or perhaps imitating so-called adult behavior.
From the child's point of view, playing with the gun was no different from previous playing with toes; it is all discovery.
Too bad too many adults among USa are still stuck at the toe-fascination level and lack understanding of cause-effect.

Why was that gun left lying around where a toddler could play with it and why was there a round chambered so it could go off?

It seems to me that the NRA and other pro-gun outfits spend more time lobbying for the “rights” of psychopaths to own military-grade assault rifles than they do for basic gun safety.

The neighborhood is not a red area.