The Buddha points out that in his day the various trades of the butcher did not pay off in living in luxory, or possessing wealth and social acceptance and that those engaged in such trades could look forward to rebirth in Hell.
Read the Sutta
Index to available translations: AN 6.18
The sutta is so worded that we can say that it is speaking of the material results of those trades in his day which is good, because today we can see that there are at least some of those who engage in those trades on a grand scale who have in fact become wealthy, though I am not sure how well accepted they are in society.
How is this explained by kamma?
Without claiming that I know precisely, I would speculate that it was a combination of some good kamma having been done in the past together with the mental focus not on the butchery but on the marketing and possibly even on the benefits of feeding people.
There is also the fact, however grim, that one engaging in the mass marketing of animal flesh is also intentionally giving life (however short) to, sheltering, feeding, and medicating large numbers of animals.
Kamma, according to the Buddhist understanding, would still entail experiencing the eventual consequences of the intent to kill or the intent to have another kill, that is necessarily bound up in this trade at any level.
There was, at one point in the recent past, an owner of a chicken ranch that distributed chickens throughout the eastern U.S. who personally appeared in television advertisements for his chickens. Anyone with vision, and children, could see his physical resemblance to a chicken and from there imagine a progression from chicken to king of the chickens to ruthless emperor chicken=human of millions of chickens while on their road to the slaughter-house while on his road to Hell for a very long ... um ... it is a pecularity of the Devil that he gives some really bad men 'enough rope to hang themselves' ... stretch.
Not so different from being king, emporor, president of a bunch of humans on their way to the slaughter-house.
There is some reward for managing the process.