His pride is most evident in Book 1 after he is thrown from Heaven in his failed rebellion against God. In an effort to rally his followers, he tells them that they are going to create their own kingdom to rule, and it will rival heaven, even though it is hell. One of his most quotable lines is that the mind can "make a hell of heaven and a heaven of hell." It is his pride that is taking him to this thought. His most prideful statment is that "It is better to rule in Hell than serve in Heaven." Here he is clearly staking his reputation on the idea of being the supreme being -- even if he is supreme only in hell.
His plans for revenge against God by corrupting God's creatures, Adam and Eve, are also evidence of his pride. He is not satisified to just make a new life and kingdom for himself, he must destroy something just as he was destroyed.