You can justify anything using the right framework. And though I agree it's possible to have empathy towards some and not others, that in itself as far as I'm concerned is a manifestation of a kind of mental illness, be that illness personal or, like the Nazis, a facet of mass hysteria.
It's not naive or lazy to think people are crazy. Everyone is to a degree.
Do you have more empathy for your parents, siblings, other half and children than for Bob down the road or Mbutu in Africa?
Having more empathy for some over others isn't odd or a mental illness, it's how we survive and protect our families.
While the example I gave is not the same as killing some and loving others the background of empathy to some, not others is. It's why we have far more empathy for the cartoonists killed in Paris than some Africans killed in the DRC.