Why does the Bible contain the words of Satan?

Answer:

If two political parties oppose each other, why do they quote their opponent's words?

The Bible contains an accurate record of numerous key events in the history of mankind. In that history, there are interactions between righteous and wicked people both in the physical and in the spiritual realm. A record of what the wicked say provides us with the ability to analyze why these people are wicked. For example, how meaningful would be the record of the temptation of Jesus if we only had Jesus' response, and we do not learn what Satan said in his attempt to tempt Jesus? How would we know the Hebrew writer was correct that Jesus was tempted in the same manner that we are tempted if we did not know how Jesus was tempted?

Hence, we find both sides often presented in the Scriptures. We not only know what Job said, but we also know how his three friends incorrectly reasoned with Job, and why their reasonings were wrong.