Understand The God Of The Old And New Testament

"Dear Dr. Sala," writes a friend of Guidelines, "I have a question. How come the God of the Old Testament, who allows killing of men, women and children, is so different from the God of the New Testament who says love your enemies? Did God change His mind, or are we talking about two different Gods? There is a big movement…that is saying that they are two different Gods. I would greatly appreciate your thoughts."