Wolf, thank you.
In answer to your question, I must first of all say that I am not entirely qualified to answer. With that out of the way….
Firstly, the clause you quoted is regarding the moiser who is on his way to do mesirah, and not post facto (the sentence before is post facto). I perhaps did a desservice by only posting Halachos 9 and 10, because much of the first 8 deal with bringing a moiser to B”D and making him pay. So I would have to assume that in a case where the moiser wants to right the wrong he is not to be killed. But I would need to study the Rambam carefully before answering you with conviction.
Secondly, the logic that GAW brings may point to the fact that the particular example you gave is not necessarily mesirah. I would say when in doubt, don’t inform, but I certainly would have to be 100% sure that mesirah applies to the specific case at hand. Damaging someone’s business is certainly mesirah, even if the chickens are smelly.
mepal, without question you are still not allowed to inform on someone just to save yourself from loss. The Rambam says this specifically.