Ah - OK - I was replying to the two previous posts which said effectively that we can't be perfect parents all the time, and all normal parents take their eyes off their children at some point.
What I meant was that in special circumstances where there is potential danger then parents need to be extra vigilant, they need to predict that accident are a hazard if you do not watch your children properly. Therefore if I was near a gorilla enclosure and not watching - indeed holding on to - my child, i would consider myself to be negligent and therefore disgusted that my neglect and incompetence in predicatable dangerous circumstances had led to the destruction of an animal.
Some times accidents happen that are totally unpredicatable or indeed as you mention a child behaves in an out of character way - and if a child is harmed that is tragic, but a zoo with huge wild animals in open enclosures?- At 4yrs old that's an obvious 'hold my hand' and 'watch like them like hawk' situation. It takes longer than a split second for a child to wriggle through a fence.