As quoted in Mengele: The Complete Story by Gerald L. Posner and John Ware (1986) on page 43:
… [Josef] Mengele’s main aim, as interpreted by Dr. Puzyna, the inmate anthropologist, was wholly unscientific:
I found Mengele a picture of what can only be described as a maniac. He turned the truth on its head. He believed you could create a new super-race as though you were breeding horses. He thought it was possible to gain absolute control over a whole race. Man is so infinitely complex that the kind of strict control over such a vast population could never exist. He was a racist and a Nazi. He was ambitious up to the point of being completely inhuman. He was mad about genetic engineering. I believe he thought that when he finished with the Jewish race he’d start on the Poles, and when he finished with them, he’d start on someone else. Above all, I believe that he would have killed his own mother if it would have helped him.
Posner and Ware offer this citation for this quote: “Dr. Martina Puzyna, personal interview by the authors, June 1985.”