Apologies if this has been mentioned before
Ok, first off, I do think Darkstar's blade was poisoned and Myrcella is dead or dying, and I think there's plenty of evidence for that, but that's for another day.
The focus here is that DS tried to kill Myrcella. It wasn't a thing of the moment, he was planning it from the start:
“The one in her own den.” Ser Gerold drew his sword. It glimmered in the starlight, sharp as lies. “This is how you start a war. Not with a crown of gold, but with a blade of steel.”
But why wouldn't Arianne's plan cause a war? Half of Dorne is itching for war, crowning Myrcella could have been a proper instigator:
Hotah paid more note to those who did not drink: Ser Daemon Sand, Lord Tremond Gargalen, the Fowler twins, Dagos Manwoody, the Ullers of the Hellholt, the Wyls of the Boneway. If there is trouble, it could start with one of them. Dorne was an angry and divided land, and Prince Doran’s hold on it was not as firm as it might be. Many of his own lords thought him weak and would have welcomed open war with the Lannisters and the boy king on the Iron Throne.
If he wanted that he just had to let Arianne's plan run its course. And why would he want a war, really? He's bored or something?
Was he following Doran's orders? Unlikely. Doran is not ready to go to war unless he has the support of Aegon or Dany. He said so himself in aDwD
Varys is the one who has more to gain if Myrcella dies:
1. Myrcella is Tommen's heir. As long as the Lannisters, Tyrells and Martells can gather around Myrcella, she's a legitimate threat for Aegon's claim.
2. If Arianne's plan had succeeded, then Dorne would have less need of Aegon. We know that Team Aegon is counting pretty hard on Dorne because
Varys wants Aegon to be Dorne's best and only option. By crowning Myrcella, Aegon and her would have even become enemies. By killing her, Varys leaves Dorne with no other options than Aegon.
3. By killing Myrcella, Varys also kickstarts the feud between Lannisters and Martells that Doran is still attempting to delay. Varys needs for it to happens now.
And by setting the Iron Throne against the Martells, Varys makes sure they turn to Aegon to protect themselves.
So the question is why? Why would DS do the Spider's bidding? What does he hope to gain from this?
Well, the one thing he wanted:
“My House goes back ten thousand years, unto the dawn of days,” he complained. “Why is it that my cousin is the only Dayne that anyone remembers?”
He wants to be a legend like Arthur Dayne, and Aegon can give him that:
He had tried his best to dissuade the prince from giving Duckfield that cloak, pointing out that the honor might best be held in reserve for warriors of greater renown whose fealty would add luster to their cause, and the younger sons of great lords whose support they would need in the coming struggle, but the boy would not be moved.
At least I convinced him to leave the other six slots open, else Duck might have six ducklings trailing after him, each more blindingly adequate than the last.
To sum up, by killing Myrcella, Varys: eliminates another claimant to the throne, leaves Dorne without options, starts a feud between Lannisters and Martells, thus making sure that they turn to Aegon
And what's in for Darkstar? A place in Aegon's kingsguard, a chance to become a legend like Arthur and forge his own reputation.
I mean, another Dayne serving as a kingsguard for Rhaegar's son? A brilliant PR move and a win-win situation
Note: When I say Varys, I don't necessarily mean him in person (although he had to be doing something during aDwD), but could also be someone working for him
EXTRA: This from the convo between IIIlyrio and Tyrion could be evidence of this:
Tyrion was beginning to suspect that a certain freckled washerwoman knew more of the Common Speech than she pretended. “My niece Myrcella is in Dorne, as it happens. And I have half a mind to make her a queen.”
Illyrio smiled as his serving men spooned out bowls of black cherries in sweet cream for them both. “What has this poor child done to you that you would wish her dead?”
“Even a kinslayer is not required to slay all his kin,” said Tyrion, wounded. “Queen her, I said. Not kill her.”
The cheesemonger spooned up cherries. “In Volantis they use a coin with a crown on one face and a death’s-head on the other. Yet it is the same coin. To queen her is to kill her. Dorne might rise for Myrcella, but Dorne alone is not enough. If you are as clever as our friend insists, you know this.”
Edited by Jon of the Dead, 27 November 2013 - 11:35 PM.