Apparently the source is this book:This guy:
https://en.wikipedia.org/wiki/Gustave_Gilbert
I can't find the original source for it, but the quote is supposedly:
“In my work with the defendants (at the Nuremberg Trails 1945-1949) I was searching for the nature of evil and I now think I have come close to defining it. A lack of empathy. It’s the one characteristic that connects all the defendants, a genuine incapacity to feel with their fellow men. Evil, I think, is the absence of empathy.”
https://en.wikipedia.org/wiki/Nuremberg_Diary
I don't have a copy, so can't confirm. But it seems to be widely available to buy online.