Ah the pack/group mentality that enables people to stand by while horrible things happen right before their eyes or in some case they actually join in. In this instance, it was standing by while a man kicked and stomped his 2 year old to death. Someone did call the police and the police when they arrived shot the culprit dead. Of course it is very possible that the child was too far gone when all these by- standers came upon the crime, but to just watch, well, it rankles the mind.
Here's how I understand pack/group/mob mentality: you the individual can be swept up into a group think in a crowd in certain situations. This can make you just stand by and watch a rape, a murder, a beating or the worst version--you can actually participate in the crimes--the lemming mind meld if you will. In the case of Sergio Casian Aguiar beating and kicking his son to death--people were afraid to step in and stop him (they say they were worried he would attack them or had a weapon and besides they had nothing to stop him with. Experts say that you become frozen and are afraid to step in because no one else is stepping in to help. This is not the first instance of this phenomenon and it probably won't be the last-but if just one person had said no and had stepped in, the two year old might not have been basically stomped to death.