How aware do you think Stannis is of his part in Renly's murder? A lot of people seem to think he was unapologetically on board with the whole thing but his dialogue after involving Renly's peach always suggested to me it was more a case of that while deep down he knows he had something to do with it he avoids thinking about it and has quite deliberately not dwelled at all on the idea of who killed Renly. I.e. he is remaining wilfully ignorant of his involvement.
That’s basically my read on it too.
Stannis’ behaviour at the parley between him and Renly and before seems to me that even if he has some Melisandre-inspired belief that Renly’s doomed, he doesn’t know when and where that doom will come about. Catelyn III, ACoK, tells us in its first few paragraphs that Stannis’ foragers have been cutting down trees for siege towers and catapults; Catelyn IV, ACoK tells us that he’s got his battle lines sorted out for the inevitable attack. From Catelyn’s outsider perspective, and that of everyone else on the field, it sure looks like Stannis intends to fight it out in battle.
Davos II, ACoK, of course, is the chapter with the best discussion of the issue of Stannis’ guilt, and I’m willing to take Stannis at his word here, given how the opening of his conversation with Davos shows how he values honesty.
Davos had come too far with Stannis to play coy now. “Last year they were Robert’s men. A moon ago they were Renly’s. This morning they are yours. Whose will they be on the morrow?”
And Stannis laughed. A sudden gust, rough and full of scorn. “I told you, Melisandre,” he said to the red woman, “my Onion Knight tells me the truth.”
In this context, I really don’t think Stannis would return Davos’ honesty with deliberate lies of his own. So when he confesses to Davos that he dreams about Renly’s murder (giving enough details for the reader to know that some part of Stannis was definitely involved), I’m inclined to believe what he says.
First, there’s how the topic of Stannis’ whereabouts at the time of Renly’s murder even came up in conversation.
“I have no doubt that Cersei had a hand in Robert’s death. I will have justice for him. Aye, and for Ned Stark and Jon Arryn as well.”
“And for Renly?”
In response, Stannis starts talking about where he was and what he was doing. That alone would indicate that at this point in time, after the fact, he feels he bears some kind of responsibility.
“I was still abed when he died. Your Devan will tell you. He tried to wake me. Dawn was nigh and my lords were waiting, fretting. I should have been ahorse, armoured. I knew Renly would attack at break of day. Devan says I thrashed and cried out, but what does it matter? It was a dream. I was in my tent when Renly died, and when I woke my hands were clean.”
He’s still talking about those battle plans. It seems from this that his expectations were to be fighting that morning, not watching Renly’s army splinter following their king’s murder. I really don’t think Stannis had knowledge before the fact - at most, I think Melisandre told him “this is some magic to assure your victory tomorrow.”
After the fact, however, it also seems Stannis knows in his heart that this was more than a dream (”and when I woke my hands were clean”), and really does not want to connect all the dots. As he summarises the matter twice in Davos II, “[Renly] brought his doom upon himself with his treason,” and in the parley with Cortnay Penrose,“the Lord of Light willed my brother die for his treason.” Much easier to think of it as divine retribution.