Did the devil make you do it?
I know God never 'makes' us sin. That leaves two things that do the sinning: the flesh (us and our sinful nature), a demon(s) hanging out in that person, or a combination of both. Let me explain...
Case #1 The enemy tempts and the flesh sins
If a child looks up at the cookie jar, and knows that there's a goodie in there, but was told not to touch it. The enemy comes along and says, "Oh, mom will never notice! Just go take one!" The child goes ahead and takes a cookie. In this case, it is the old flesh sinning. Sure it may have been the enemy who did the tempting, but it was the flesh that actually sinned. If the enemy was involved, it was through external tempting (he's on the outside doing his work).
Case #2 An iniquity exists
A iniquity is a weakness, sort of like anger or lust. I believe an iniquity can sometimes 'overwhelm' the soul into sinning. I like to think of an iniquity as "internal temptation" (the enemy is now able to tempt from the inside). A believe a lot of people who commit rape would have some iniquities built up in them that would give them such a strong urge to do such a thing. NO, I am NOT saying that the devil makes people rape, but what I am saying is that when an iniquity such as lust exists, and we continue to feed it, it grows, gets stronger, and it allows the enemy to put such a strong temptation on a person that it makes it hard on the person to resist living out those strong urges, etc. The person who raped is guilty of the sin, but he had been subjected to such strong temptation that it made him hard to turn it down. The solution is to break that iniquity, which is one of the reasons Jesus came and shed His blood for us. He was bruised for our iniquities - Isaiah 53. An iniquity is kind of like external temptation, except it's coming from the inside. The enemy has access into that person's soul (thoughts, emotions, feelings, etc. realm), and he's able to tempt us from the inside, which makes the level of temptation MUCH stronger and more irresistible.
Case #3 The "Devil made me do it!"
This is a more extreme thing. I read a story once of a serial killer who said that the devil truly 'made him do it.' Something "came over him" and he just couldn't resist killing. He murdered about 600 people, and Satan protected him from being caught. He would sit across the street and watch as the victims were talking with the police, and nobody would even notice him standing there! There's also extreme situations where a person can be demon 'possessed,' and wakes up in the middle of the night, goes to a coven (group of 13 witches) meeting, does a whole bunch of ritual stuff, worships Satan, etc. and goes back home before the sun comes up... goes back to sleep, and wakes up, knowing NOTHING that happened. That is obviously outside the control of the human will or flesh.
An example of this kind of 'demon possession' activity can be found in Luke 8:29, "For Jesus had already commanded the evil spirit to come out of him. This spirit had often taken control of the man. Even when he was shackled with chains, he simply broke them and rushed out into the wilderness, completely under the demon's power." (NLT) Some other verses to look up include: Matthew 8:28, Mark 5:5 and Mark 9:17-18.
My dad said that there's two men on your shoulders, and whichever one you listen to will crowd the other one out. If you live a careless life, and make a habit of sinning, you could open yourself up to iniquities and therefore you give the enemy room to tempt you from the inside, therefore the temptations can be much stronger and more irresistible... perhaps to the point where you can barely say no!
If you want to learn more about breaking free from iniquities, check out Larry Huch's book titled Free at Last. It is AWESOME!