I wonder if what he did last night, i.e. strip The Mountain of everything, was him losing the plot a bit. Was he truly carrying out the King's Justice, or was it his own? Surely what he's done is abuse the King's power to further his own goals, rather than the right ones? Or maybe he sees the Lannisters (And perhaps rightly so) as the cause of a number of issues, and if he rids Westeros of them, perhaps there'll be less problems and fighting between the clans? I guess, in an odd way, Ned Stark is actually very naive. Edit: I forgot - Tyrion confessing the crimes had me in fits of laughter.