Help to to realise what the consequences of our actions may be and give us the strength to do your will.
I'm going to start with this one as it is a fairly easy one to deal with.
The concept of a devil or Satan can be traced back to various pagan mythologies. Probably the most relevant is the Celtic belief but many pagan religions have some sort good vs. evil power struggle. It made the Greek fables and bardic sagas a bit more interesting!
The original biblical texts didn't have many references to 'Satan' but these where added by the translators for the following reasons.
The ability to blame someone else sounds quite a childish reaction, but if you look around, especially at ourselves, we see just how often it is done. Just look at politicians (see, now I'm doing it!)
The concept of 'we're right, you're wrong' has been used for millenia, often to encourage people to fight wars in the name of religion. The Crusades was a perfect example - killing an enemy was considered a 'good deed'. The logic went along the lines of "they're supporters of Satan, aren't they, so we must be right". This argument was used by both sides, and in some ways it is still used today.
There is also a secondary aspect, in that accepting that there is some immortal who can challenge God diminishes the fact of God's omnipotence.
To Summarise: the Bible uses the title 'Satan' where it means adversary, someone who tries to thwart another person. The most obvious example of this error is that when God decides to test David (recorded in 2 Samuel 24:1 and 1 Chronicles 21:1) - the mediaeval translators actually call God 'Satan' in the Chronicles account!
How does accepting responsibility for our own actions influence our lives?
What can we do if we are falling into the trap of blaming others?
What effect does this have on how we ask for forgiveness?
How can always justifying all our own actions alienate us from God?
Can you think of any biblical examples?