Without knowing the details, it's impossible to say. Maybe there was no one around with the physical capacity necessary to do the job. The witnesses could have been a bunch of old people and some 90 lb Thai women. Maybe it was the "gruesome witness effect" where everyone is so paralyzed by a horrible sight that no one can act. Maybe they tried to stop him and he was simply too crazy to drag him away effectively. People can be really strong when they're totally nuts.
More than likely, the baby was already obviously dead within seconds. I don't know about you, but I'm not going to go fight a PCP maniac over a dead infant unless it's mine.
All I'm saying is, don't judge people unless you know the full circumstances. You might think someone is a worthless coward until you're in their place.
This is obviously terrible though...not just the death of an infant, but the emotional pain of all who knew them, and the fact that the baby got cheated out of a life. The main thing to take away from this is (1) intervene if you see something bad happening, and (2) if you have any mentally ill family or friends who becoming ragingly violent, keep them away from children.
+1 Well put Toreskha, but I do disagree about the "90 lb Thai women" part. I've seen many Thai women in the 90 - 100lb range who would be able to put a guy three times their weight on their back.