From the story it sounds like he didn't reallty even pay much attention to the people trying to stop him. If he was high on PCP, you probably could hit him all day with small effect. As to 21 bullets, depends on the locations of the wounds. Head shots have a much higher mortality rate. And the size/age does make a difference to me. Two adults may have been having a mutual altercation prior to witnesses arriving and while not justifying the beating, at least explains a bit of it. A grown man pounding a toddler is significantly different. The unarmed child is no real threat to the man and never was a threat. Pure stronger person against defenseless child.
This is a call we all hope never to have to make. We each have our own experiences we use to make our judgements and have to live with the results.
I would have a hard time living with myself if I did not do everything I could to stop the attack. I hope I would jump in there and do something.