I'll quote a post from another thread because Herber Book List explains it well:
You are reading into the account that which is not there (eisegesis). It does not mean that the King of Tyre was, literally, an angel. Ezekiel uses the imagery of an angel to describe the King's declared greatness, before his fall. Verses 12 - 13 in the Hebrew text are set as questions: 'Were you the Seal of Perfection? Were you full of wisdom and flawless in beauty? Were you in the Garden of Eden? Yes, you walked on the mountain among the fiery stones' (refers to David and Solomon). In short, the King was trying to out-G_d, G_d, by the way he saw himself as very much equal to, or even as being better than G_d, and the way he managed the affairs of State, and so these verses are meant to be sarcastic* as are the other parts of the text, before his fate is declared.
*Some versions of the text, because of this sarcasm, are written as positive statements, not as questions, again, simply reflecting back to the King how conceited he was and, for that reason, it is put in direct contrast to the enormity of his downfall and punishment.