I'm very pragmatic and sanguine about this, possibly partly because I do get to put down animals in either their best interests or very occasionally for general safety.
I'd put down a dog that had bitten a child without compunction, regardless of the circumstances, because the greater good must prevail and children must be protected and looked after.
I cannot imagine anybody with the merest remnant scrap of decency being happy in the knowledge that they were abusing children...therefore, either put them down because they are miserable and unhappy in themselves, or put them down because they have not a shred of decency in their soul.
If we can't bring ourselves to that, then a lifetime's incarceration somewhere secure and forgettable.
Either way, the greater good is served and potential future victims are saved.