I don't know what to make of people who say things shortly before death that seem inconsistent with the life they led. He was 94 when he died, and wrote that letter not long before that.
The dude was a general. His entire career was centered around killing in a legally sanctioned manner. Apart from his invention, his life's activity resulted in many deaths over and beyond his invention.
Not suggesting he wasn't sincere, just pondering on the turn of his thought and perspective at that particular stage of his life.