I think Szeth's gaining of the fifth ideal, and specifically the manner in which it was sworn are important.
"I AM THE LAW".
The nuance here, to me anyway, is that he is saying that he is capable of deciding at any given moment if something is legal or not, and when both he and Nale (brokenly, I'll admit) have reinforced to each other that the law is what is right, this holds some measure of importance for Honor too.
Honor is currently portrayed as a child who doesn't quite understand right from wrong, and that the words are what truly matters. But Dalinar tells Honor to go with Todium and "learn". I think Honor will itself, over time, come to understand that while the words are important, that it itself is capable of deciding if something is right or wrong in the moment, and whether an oath is upheld, broken, or even made more or less valuable by the actions taken to either uphold or break it.
If someone is capable of reaching the fifth ideal on this premise, then I think Honor itself can learn too.
Szeth isn't just forshadowing Dalinar's choice, he's also forshadowing a change in Honor and/or Retribution in the future. It's likely that Honor will leave Retribution when Big T does something that's so incredibly wrong, even though he's upholding an oath.