Let's look at the Devil's situation:
He's lost hisstatus as top Angel and he blames it on Adam, so in his mind Adam's line must suffer. Satan thinks that if he is dammed then man might as well be dammed with him, he would do this rather than repent because he is proud and spiteful.
Since the Devil is a spirit, he isn't solid in this world so he can't hurt humans physically but he does his best to give them misery where ever he can. He will do anything to lead a person away from God and the closer they are to God, the greater his satisfaction is when they are lead astray. I think the holier a person is, the more he wants to turn them away from God and the bigger the victory it is for him.
Look at what he tried to do with Job for example. He wanted Job to crack under the tests that God gave him and eventually turn against God.
The devil is also very subtle and sneaky, he doesn't go around with horns and cloven hoffs, stomping about and scaring people to death. He wispers in your ear and burrow's himself right down in your subconsious and feeds you negative thoughts to make you miserable and cloud your mind, making it harder for you to see God.
Has anyone read; "the Screwtape Letters," by C.S. Lewis. I would highley reccommend it.