I never reared the criminal and have no knowledge of the crime whatsoever. Nor was I old enough to take care of someone. Nor did I encourage the kind of environment that help cause the criminal acts. Nor did I endorse the punishment of death.
It is easy to use "we" to name collective responsibility, but you must also remember that behind each "we" is a group of individuals.
Society does not act, move, and feel, have goals, beliefs, etc. It is a heterogeneous collection of individuals, each with their own belief system, predisposition, history, relationship, and other distinct characteristics.
Society can be more understood as a force, which its sum is greater than its parts. Yet, we also know that society cannot exists without individuals.
Ultimately, responsibility alway lies with the individual(s).