Expaining The Holocaust & karma to a non-Buddhist

How can you explain karma in the genocide of babies and "innocents" (For example: The Holocaust) in Buddhist terms to a non-Buddhist, without seeming cold and uncaring? Yesterday, I tried--by saying, "I cannot say unequivocally, why the millions had to suffer or whether karma came into play or not, but perhaps it is better to focus attention on the present and try to generate good karma here and now." I know Buddha said certain topics will only foster disputation and I felt like this is one of those that can only be speculation so I steered the conversation in another direction. What can you say in a better (or kinder) way to explain it to a non-Buddhist person who asks how the holocaust suffering can be justified and who expresses interest in knowing the Buddhist view about it?