In "Word Focus" William Sillings writes:

The text says that this man had an unclean spirit (pneumati akatharto — a spirit, an unclean one). Another way to describe this unclean spirit is to say that the man was mad. According to the Talmud, there were four signs of madness — walking abroad at night, spending the night on a grave, tearing one’s clothes, and destroying what one was given. This man had the added characteristic of unnatural, almost supernatural strength. He tore the chains and shackles that had been used to bind him, and no one was strong enough to subdue him (v. 4). Add to this the fact that he was self-destructive and often cut himself with stones, and you have a classic case of extreme demonic possession.

While this definition of demon-possession is intriguing, Sillings' next paragraph exploring the fact that this man had no control over himself is more important...

His name was Legion, he said, for we are many. The word legion is the name of a Roman army unit containing about 6000 soldiers. Even when this man spoke, it was not a man that spoke, but a multitude of demonic spirits speaking through him. What could this person have done to have become so possessed with this multitude? It is clear that the man had not the slightest control over himself, his thoughts, his actions, his words, his feelings, his strength, or anything else. He was a pitiful case. Anyone who is in any way bound by sin is, in some ways, in a pitiful condition, but this man was hopeless.

Discussion: Do you know anyone who, while not demon-possessed, seems hopelessly out of control, spiritually? What is our hope for such hopeless cases?
Source: Miracles of Jesus: Adult Teacher's Insights, page 52.