The author start out by telling us that if we let any explicit rule be violated regularly this will lead to lack of moral guidance.
We then learn that lack of moral guidance will lead to chaos- so we have a bit of an inference chain here: routine violation to LMG to Chaos.
The author then concludes that because of this chain, we should never allow any rule to be broken with impunity.
There are two potential issues here. First off, the "should" is slightly stronger than the argument warranted. The argument has shown that allowing rules to be routinely violated would lead to bad chaos, but what if not allowing them to be broken would lead to something even worse than chaos?
There might be another issue here, which is that impunity is not the same as routine. Routine violation implies that the rule is regularly violated many times, while broken with impunity means that it's a particularly flagrant violation. It is possible to have a rule rarely broken, but broken with impunity when it is.