Why is the Red Wedding considered such an atrocity given that Robb was willing to abandon the riverlords and all their smallfolk, vassals, etc to certain death at the hands of the Lannister-Tyrell alliance? It appears to be a logical self-preservation that saves lives in the long run.

I feel like I’ve covered this one before:

1. the Freys decide to betray him long before Robb makes the decision to retake the North. So there’s not really a two-rights-make-a-wrong argument here.

2. the Freys murder thousands and thousands of Northern soldiers, along with the wedding guests. So it doesn’t really save any lives. 

3. the Freys make war on their fellow Riverlords, including their own liege lord, and seize their lands by conquest. So I don’t think Riverlander nationalism can really been invoked here. 

4. the Freys violate guest-right, one of the few customs that keeps Westeros away from the war of all against all. So no one bats an eye anymore when people murder Freys indiscriminately.