After re-watching the crossbow scene on YouTube (nudity and all weirdly enough), I actually wonder if Littlefinger had a hand in this for retribution for Ros and making his own moves since this is definitely his sort of play. Also, the drunkard is helping Sansa escape which we know how Littlefinger feels about her and with Catelyn dead, maybe he figured it was time to finally get her out of there assuming he ended up at The Vale. I mean, he's been absent from the show after being quite the crucial character so I just wonder if he's involved due to his feelings for Catelyn and Sansa, his hatred of Joffrey, and his cleverness lends itself to an assassination like this with so many patsies in front of him that no one would suspect him.
Based on the scenes with Olenna though, I'm sure she's involved as well. I originally said this was the type of move Varys or Littlefinger would be proud of, but we've seen Varys lately and I don't think he'd care about Sansa while Littlefinger has been suspiciously absent after sailing away and he would love to protect Sansa as well.
Maybe it's just me wanting Littlefinger to return since he was such an interesting character, but this assassination is right up his alley and he has reasons to do it as well.