Why does the God of the Old testament seem so different from the God of the New Testament?

While I am far from religious, I found the Cathars' interpretation of the Bible to be enlightening when compared to the modern business dogma that seems to drive most modern churches. The Cathars believed that there were two separate gods--the one of the Old Testament and one of the New. The god of the OT in their eyes made the world a prison and spent most its time demanding blind obedience. The god of the NT was about liberation from the world and love. They believed that this world is Hell and we have nothing to fear after death except rebirth back into the world. Unlike the Catholic Church, the Cathars practiced what they preached and that might be the reason why the CC pretty much destroyed them.