Humans have always been like this. The question you pose is answered beautifully in Sinners (Season 3), if you enjoy psychological dilemmas, it is a good one, and since you review shows and movies, it would make for great research as well.

Humans are now bound by social norms, but they were always on a spectrum. Some behaved within those norms yet felt as though they were not truly alive. But as they ventured deeper into the macabre, they found it fun, mostly in observing those who blindly followed orders. Killing is not something they considered abnormal, as it was quite commonplace in older times, no investigations, no consequences.

Many criminal documentaries and books on serial killers that emerged from the 1980s reveal that these individuals moved like no others, the law could not get a hold on them. They were ultimately caught by technology that had never even entered their minds, and most were only caught toward the end of their lives. So in their time, they had ways of going undetected that could very well hold true even today.

The incident you mentioned can stem from social abnormalities, in some countries people kill their own offsprings for things like that.