The Christadelphians teach that the devil/Satan is not a personal being but an analogy for describing sin.
What is their Biblical defense for such a theology?
The following paragraphs are copied from http://www.religioustolerance.org/chr_delp.htm which in turn cited the page titled "What Christadelphians believe about the Devil" at the Christadelphians's official website (http://www.christadelphian.org.uk/wcb/devil.html)
The Devil is not viewed by Christadelphians as a quasi-deity with magical powers who travels the Earth trying to lure people into sin --as is believed by most conservative Christian groups. Based on Genesis 6:5, Jeremiah 7:21-28, Matthew 15:19, James 1:13-15, and other passages, they stress that "Temptation and hence sin, comes from inside the person, not outside." Satan is viewed as the principle of evil which resides in people and motivates them to sin and rebel against God.
They point out that the words "devil" does not appear in the Hebrew Scriptures (Old Testament). In the New Testament, these terms are sometimes used to translate the Greek word "diabolos" which means a human slanderer or false accuser. Examples are John 6:70, 1 Timothy 3:11, 2 Timothy 3:1-3 and Titus 2:3.
"Satan" in Hebrew and "satanas" in Greek means an opponent or adversary. Examples are: Matthew 16:23 when Peter was considered an adversary because he opposed what Jesus wanted to do at the time. In Acts 5, Sapphira was an adversary.