@Cyan:
I don't even think Tywin believed that "12 men at dinner" line, considering his past history (goodbye, Reynes and Tarbecks!)
He believed to the extent that it saved the lives of his soldiers and thus the already-stretched Lannister military strength. Maybe it's a bullshit line no one really believes, but it's not entirely false.
Meanwhile, Stannis immediately spent the lives he "saved" by killing Renly on a failed attack on King's Landing, one that probably would have succeeded had he not taken the detour south to Storm's End. Holding the Red Keep would have been a different task, but he still would have had at least one shadow baby in his loins and/or lifeforce to take out the first commander to arrive.
–- Update From New Post Merge ---
@maxterdexter:
Renly's was more in self defense, while the red weding was needdesly cruel.
Renly was his death and a couple of guards that Loras killed, while the red wedding was more than half of the northen and riverland army, saving only a few valuable hostages, and the soldiers that could manage to escape the carnage.
Renly's was in the middle of planing how his soldiers should kill his brother, while the red wedding was in the middle of repairing a broken oath.
Renly's was his brother, while the red wedding was his king/king/mortal enemy (bolton, frey, lanister)
Stannis regretted that act ever since (the whole peach talk, replaced in the series with the "do your dutty), while the red wedding is an act of pride of the other men.
Excellent points, but our perception is also somewhat tainted by our stronger emotional attachment to the Northern characters. It's also tarnis he'd by the characters associated with each act. I think book readers will agree that the Bolton and Freys are more unsavory than Melisandre, who at least in her own mind thinks she's doing good.
Just playing Devil's Advocate here. I agree that the Red Wedding was a worse atrocity, though I wonder whether the defiling of guest right is worse than kinslaying in-universe.