I know this is season 8 talk but I watched season 2 ep 1 and the scene where Littlefinger subtly threatens Cersei about "knowledge is power" and then she has her guards grab him, tells them to slit his throat then giggles and says "oh I changed my mind ..." Right there, I was like, oh that's in part why he decides to kill Joffrey - he's embarassed, insulted and a man like that, yeah, he wants to get back at Cersei ... the best way, through her children.
Of course, that could all be shite and just my rampant imagination ... :laughing