This is a Chaos Space Marine renegade. I was reading Realm of Chaos: Slaves to Darkness, and noticed the pictures of 'Chaos Armour,' particularly the one on page 252.
I don't know what went wrong, but for some reason, Games-Workshop abandoned the organic, almost Giger-esque looking armour in favour of plain looking armour with spikes on it. I bet it had something to do with Jervis Johnson. Chaos is a strange power, and the gift of chaos armour should reflect the extra-dimensional essence of their deities.
This one is of the World Eaters legion, who according to the old fluff, still viewed themselves as righteous. It was the 'False Emperor' and his other chapters that have sunken into decadence and depravity.
Too many hours to count. HB mechanical pencil, GIMP.
Comment! Or die with your corpse-emperor.