Wodium made a comment about 'ineffable' reasons for a Chaos god to cause trouble. Let's see if we can come up with a few to flesh that out.
1. Spite: Whatever the Creator made, He likes. I don't like the Creator, so I'll destroy his stuff just to make him unhappy.
2. Humans are boring, nine-legged chimera with bat wings and laser shooting eyes are SOOOOOOO Much Cooler!!
3. The Bible describes Satan as a lion going about seeking whom he may devour.
A. Devour, like a vamp does. "Mmmm, tasty mind, so much power."
B. The devourer has a hole in their heart. (In Krillis' case when he went evil for a bit--caused by fear). Only sadistically causing the fear in others eases the pain of your own fear.
4. Jealousy: "Why does the Creator like these stupid humans better than me? I'm a being of great beauty and power and wisdom."
5. Disgust: "How sickening! Beings of mud and immortal fire? Being in their presence makes me want to hurl. I fail to see the beauty the Creator ascribes to them, and it makes me question His artistic judgment."
6. Ransom: I'd like to do thin in a novel. A patriarch, and great-grandson of Adam, is five hundred years old and has hundreds of thousands of children (not sure of the exact figures, but one guy pointed out that you could have 2 Billion population on the Earth before the Flood.) He's, in Multiverser terms, an Expert on a wide array of topics, and he's got a lot of charisma. So when he tells his great-grandkids "I'm God" and demonstrates his Divine Power by shooting a mammoth from a mile away with a giant longbow, and killing it with one shot, his great-grandkids believe him.
His logic is that God loves his great-grandkids more than he does, and so God will be willing to make a deal.
7. Angeli a Deo: Not mano a mano, but someone thinks they can take the Deity in two falls out of three. Ambition. Power. Greed. Desire to be worshipped rather than worship.
A. One book I had pointed out the possibility that Lucifer knew God as the King of Heaven, and did not realize he was Omnipotent. In other words, God kept back a trump card. This allowed God to see if Lucifer would be loyal out of love and respect and duty, or was only being loyal from fear.