Also, it's best to strip the username and post ID# from the quote, cause apparently there's a feature of "who quoted you" that they could see in their control panel.
[series]It is Joffrey, Tyrion thinks at lengths about this. Remember that Joffrey, while Robert wasn't his bio-dad, was very much influenced by him. Robert offhandedly remarked that Bran was better off dead and it'd be more merciful to end his misery. The shiny expensive dagger also belonged to Robert's stash, which Joffrey would have had access to. Tyrion guesses that after Joffrey brags about being "no stranger to Valyrian steel", and he mentions the dragonbone hilt dagger and Joffrey gets all flustered.
ah yes... that evidence. Still doesn't make sense for that character to do it.
mercy doesn't really seem to be where he aims.