Who did it?
Don't click and read the following link if you don't want to know who I think did it. There was a clear opportunity to pursue it and there was definitely motive. A minor difference between the TV series and the book, but in the end it does not matter. Page from A Song of Ice and Fire Wiki
BTW if you want to know what is ahead, that website can give you some insight.
FYI, not a spoiler as if you watched the show you can probably figure out he got poisoned, the method: Tears of Lys