Sure, there is a "legal interpretation" of responsible.
But anyone who leaves fire power like that unsecured is simply irresponsible by the definition of the word itself.
For instance, it's not technically illegal to leave your car running in your garage with all the doors and windows sealed shut while you work on your car, but it's very irresponsible.
Anyone calling what Ms. Lanza did "responsible" has lost credibility or doesn't know the meaning of the word, plain and simple, regardless of any invented strawmen arguments.
'13 Audi A6
'07 Porsche GT3 RS
'08 BMW E90 335i