In your analysis of Cat 3 in ACOK, you mention that Renly is possibly more guilty of kinslaying than Stannis. Kinslaying, like breaking guest right, is one of the gravest sins in Westeros, but is it’s taboo affected by war? Namely, if Stannis and Renly met on the battlefield or in single combat, is it still viewed so badly?

Judging from Maekar and Baelor, yes, war counts. 

The reason why I say he’s possibly more guilty is that he decides to fight in no small part to kill his brother and wipe out his claim, and gives orders about what’s to be done with the body. 

That’s more mens rea that Stannis seems to have, given that I don’t think Stannis gave the order to Melisandre to kill his brother. I think she did that to try to change her prophecy.