He attempted to marry Margaery to Robert because he realized the Lannisters were not good for the realm.What are you talking about? Renly's actions make him a real scumbag and of course he wanted to be King, he says so himself.
-He first attempted to marry Margaery to Robert even going so far as asking if she looked like Lyanna to Ned's face. This is a man who was trying to manipulate his brother into putting aside Cercei and marrying Marg by trying to convince him that she looked like his dead love. Him planning this was likely an attempt to gain more power since the Tyrells would be grateful to him for arranging the marriage.
-When Robert dies he attempts to steal the throne from Joffery who for all his knowledge he believes to be the rightful heir. He's not doing so to protect Joffery but so that he can rule through the boy, essentially holding him hostage.
-When Ned refuses he flees the city, marries Marg himself, and declares himself King all the whole still not knowing that Joff is a bastard and knowing full well that his brother Stannis is the Elder.
-When he speaks with Catelyn about his claim this is his response:
-When treating with Robert this was part of their exchange:
And the final line which proves that Renly fully intended for Stannis to die during the battle:
Renly may have been an alright King, but he was a dick and expected Stannis to die during the battle so he could usurp him.
He was not planning to become king and that was not his initial end goal. He never would've tried to kill Robert. In fact he was ready to aid his nephew as king.
He tried to marry Margaery to Robert and was aided in the fact that she might have resembled Lyanna. That's not a devious cunt trying to usurp power. He was trying to dispose of Cersei, who, by all accounts, was one of the most beautiful women in the realm. Best way to help that? Find an equally beautiful girl who happens to look like the love of Robert's life.
He attempted to remove the Lannister influence from Joffrey's rule. He asked Ned to join him in the task. That's not him being power hungry. Asking Ned implies he'd not rule over the realm by himself, he legitimately just wanted the Lannister's out.
He put himself in danger gaining the alliance and declaring himself king. Why should he have then fought to put Stannis on the throne. Everyone agrees that Stannis would have been a shit king. He was not fighting on the base that Joffrey was illegitimate. Had Stannis brought that to light earlier, maybe things would have been different. But would the Tyrells be happy marrying their daughter to a Prince? I doubt that.
And you quoted my favorite line of his in the book. There were at least 4 people ahead of Robert in line for the throne. The mad king was cray, but Rhaegar was not, Viserys and Dany were children. I believe even Jamie thought that Viserys would be the next king, as it was his by right. But Robert claimed it by conquest. Why should Renly not do the same?
Oh by the way, Stannis knew that Cersei's children were illegitimate, but ran off and his argument was that it was because "Robert hates me".