Martin has stated he wanted to show that war has consequences and that “the good guys don’t always win.” The Red Wedding was a deliberate response to the trope seen in Tolkien (e.g., the charge of the Rohirrim saving the day at Helm’s Deep). Martin wanted to show that even a heroic, justified war (Robb avenging Ned) can end in brutal, anti-climactic failure due to broken social contracts.
In all cases, the perpetrators justified their brutality as a "necessary evil" to end a war or secure a throne. 💡 The Legacy of the Red Wedding what was the red wedding based on