A very long time ago, I bought some random Chaos bits off ebay. One of the bits was half of a Chosen space marine from the old Dark Vengeance boxed set. These models came in two bits, presumably for ease of manufacture. Unfortunately, I was missing the bit that contained the marine's forearms, head and most of his shoulder pads.
I decided to rectify this by giving him new arms and a whopping great rotary cannon. I cut and repositioned the legs so that he would be posing dramatically with one boot up on a rock. He got new arms from an ancient plastic space marine, a new head from an old chaos champion, and a lot of green stuff and plasticard to bulk out the missing pauldrons.
(This, I should add, was really hard. Green stuff is fine for filling gaps, but I find sculpting with it very difficult. I ended up using some tips from this excellent and under-followed Borgnine's Painting blog. In particular, I was careful to let one load of green stuff cure before trying to do another. I'm still not 100% happy with my efforts.)
His gun was built out of half of an ork autocannon that I had lying around (as you do) with an ork nob's magazine and a barrel made out of an Empire outrider's gun (thanks James F). I think it looks suitably excessive, as if he nicked it off a helicopter gunship.
And now with paint. I went with my usual not-quite-Black-Legion colour scheme, with slightly more gold in this example to reflect his higher status (purely on the basis that he's got the big gun).
Here he is next to the model from which he was converted.
I've never really seen chaos marines as evil: everyone in the 40k world is a knob (except ork nobs, who spell it differently). It's more that the chaos marines rock hard and do what they like, like pirates, while the normal marines are more boringly obnoxious and follow orders. Anyhow, this guy will be rocking very hard.