This is how I see these 2 cards:
The eight of Swords is about not being able to think straight. It's about having so many thoughts in your head that you don't know which way to turn.
It's that white-noise you get in your head that you make worse by going over and over things.
It's crippling indecision or it's overanalysis..
Eights are about balance, they relate to Justice, the answer is there in the card - you need to strive for some mental balance, organise your thoughts so that you can see the way out of the mess.
The Devil on the other hand is archetypal; he's The Dark Side
Everyone's got one...
He's the things that we all do that we hate about ourselves for or the things that we do that we know just feed our own insecurities and are really bad for us.
The yo-yo dieting, the way you know that you're nasty to your mother but you just can't help it, the fags that you really want to give up, the 'recreational' drug use that you know you can't afford and is edging nearer and nearer to an addiction, the men who treat you badly but you always seem to go for the same type...
The Devil is the flip side of The Lovers; he's the voice in your head that tells you that you have no choice.
The Eight of Swords is the way that we tie ourselves up with our own thoughts, worries and insecurities.