Board Thread:TV Show Discussion/@comment-4708902-20161003211805/@comment-28895263-20161012004744

Eko is Oke wrote: Littlefinger is too smart for everyone. The only one who can really outsmart him and beat him (kill him) is Varys!

There's bound to be a possible confrontation between these two at some point. Varys was my first thought for Petyr's killer, too. Varys is traveling with Daenerys' court; if there's a diplomatic mission to Winterfell, he could certainly be on it. He wants Tyrion alive; if Petyr wants to kill him (leaving Sansa a widow, and available), Varys has motivation to prevent it.

(I'll freely admit that my enthusiasm for Sansa as Petyr's killer stems from my desire to see her avenge her father's betrayal at Petyr's hands.  Varys is a far more believable choice.  No closure for Sansa, though.)

Varys wouldn't use a blade. Varys would likely use poison; perhaps obtained from Tyene Sand. We've already seen Petyr use poison, of course. Perhaps both Petyr and Varys die, victims of each others' poison.