When he was younger, Bennett was implied to be very depressed as a result of the tests run on him. As a result, he was incredibly motivated to get a promotion away from it, even to the extent of collecting "motivation points" by reporting people, knowing they wouldn't come back. When he had gotten his promotion, his depressing state only worsened and he appears to have given up on trying to escape the constant testing. When he met Huxley, he was blunt and sarcastic, calling the latter a "madman" for his plans and saying he would rather be dead than go with him.
Hello Charlotte

(copied from wiki)