First of all, as if roach feces stains aren't bad enough, the stains are probably from roach spit up, too! (That's what causes most stains.) No matter what end of the roach it came out of, we're talking about nasty stuff. Roach "debris" is also a major allegan and can trigger asthma attacks.
So you should really clean it THOROUGHLY before you paint. First clean and scrub with a detergent solution, then toss the rags and detergent water. Then wash with a bleach solution and rinse. After all that dries you can move on to painting.
If you start with a primer designed to hide stains, like KILZ
or Cover Stain
, that should cover any remaining stains.