I agree that his irresonsibility contributed in some measure to the situation these unfortunates found themselves in, but he did not beat them, torture them, and finally kill them. He just left them to be be beaten, tortured, and finally killed.
So yes, he is as big a POS as the deviant sadists who snuffed the life from that child, but his was a sin of omission. He was notable only by his absence.
Sins of omission can be fatal that way.