His thinking became warped. God gave him and the angels control over their own minds. They can never straighten them out - never again think rationally, honestly, rightly. I have had a number of personal experiences with demons through a few demon-possessed people. I have cast out demons through the name of Christ and power of the Holy Spirit.
Some demons are silly, like spoiled children.
Some are crafty, sharp, shrewd, subtile. Some are belligerent, some are sassy, some are sullen and morose. But all are perverted, warped, twisted. Lucifer was the supreme masterpiece. If Lucifer and his angels went wrong, there was no assurance the other two thirds would not.