This brother was very troubled by spirits, especially anger, hate and rejection.  He had suffered a very difficult upbringing and many people had rejected him.  The spirit of anger then entered him because of rejection.  So we prayed to expel the spirit:

When we feel rejected, sin will desire to enter us and take over:

Genesis 4:6-7 King James Version (KJV)And the Lord said unto Cain, Why art thou wroth? and why is thy countenance fallen?

If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.

 

Even though sin tries to come in, we are to rule over sin.  We must prevent it from coming in and taking over us.  If it does come in, we must expel it from us.  This means we must remit (transfer) the sin from us.

John 20:23 King James Version (KJV)

23 Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained.

 

Once we understand what sin actually is, we can then understand that casting out demons is the remission of sin.  And this is the gospel that we are called to preach:

Luke 24:47 King James Version (KJV)

47 And that repentance and remission of sins should be preached in his name among all nations, beginning at Jerusalem.