it's not the desomorphine that rots the skin.  desomorphine was actually commercially sold under the name Permonid for quite some time.  however, junkies don't have lab grade desomorph.
when junkies synthesize desomorphine from codeine pills, they use some nasty chemicals in the process...gasoline, paint thinner, hydrochloric acid, iodine, and red phosphorus.  the end result is by no means a clean substance...they don't filter out the impurities, etc., they just shoot it up.
 yes, you can add the detail that they miss their shot as part of the process, but i didn't think that detail was all that important as ultimately it's all of the nasty shit mixed in with the drug that causes the skin rotting...not the actual desomorphine itself.