... Lysa is looking at him while he has kissed Sansa?
At first, sorry for my English if something will be incomprehensible.
I mean, he is a mastermind with a couple steps ahead of everyone. He has planned marrying Lysa long before everything started (he knows her love for him, he is completely sure that would be part of his interest at some point). So I'm wondering how could that be possible that sweet Petyr, who shows us propably the best adjusting to the environment in various situations could expose himself to Lysa's anger on purpose because of irrational kiss of Sansa? He must have known she was looking to find a way to get rid of her as soon as they got married! I'm reading a lot of comments like "creepy Littlefinger kissing Sansa, gross!". That was his plan all the time, not being Pedofinger. Always keep your foes confused. If they are never certain who you are or what you want, they cannot know what you are like to do next. Sometimes the best way to baffle them is to make moves that have no purpose, or even seem to work against you. Remember that, Sansa, when you come to play the game. 1. The gate to the room is closed, so nobody can step inside, especially guards.
2. There is a person to frame with high influence to Lysa.
3. There is a motive for framing Marillion.
4. There are no witnesses
5. The goal is achieved.
The only one thing Petyr cannot control was what Lysa has said in anger (about Jon Arryn, the letter, etc.). It's exactly the same situation when Ser Dontos said the name of Kettlebeck on boat after Purple Wedding, and Oswell answered back: No names.
And I come to the conclusion that if Littlefinger has to die, it would be because of such LITTLE DETAIL like above.