Walder Fey did something terrible, his lot are terrible and there fickle, but i can relate to him, being cautious first (thus earning the name the Late Frey), at the time the dragons still ruled, so his decision then was more difficult then now, to me, it seems, to choose sides (they (dragons) had been ruling for many hundreds of years by then, whereas the Baratheons reign just began).
Moreover he has a reason to be angry at the Starks,add to that the consideration that the North was clearly on the losing side at the time, i find his actions somewhat understandable. I mean why drown with a sinking ship when a lifeline is given.
Dont get me wrong what he did was terrible, and it is understandable to hate him, but if i had to choose between him and the likes of Cersei Lannister, who in my eyes has done a lot worse (you can make a page-fulls lists with all she did wrong, and many in an as large if not larger scale then Walder frey.)
Jofrrey is hateful, but he is still just a idiotic kid as well, who owns his demeanor a lot to Cersei's upbringing. Therefore:
I remain at my standpoint cersei must die, wich is why GRRM might actually let her live. (though there is this funny volanqar prophecy though, not sure if that means she is actually gonna die, seeing that the words are formuled so strangely, that and i dont remember them exactly
)