You see, I don't believe it is - I think it's traded on the scene in Dagobah for far too long, the one where Luke chops off 'Vader's' head revealing his own face. There's not really much else that's particularly 'dark' in the film - the violence in ANH is far more disturbing and graphic - Vader crushing the guys throat, Luke's Auntie & Uncle getting BBQ'd, severed arm, the torture droid (which is also a feature in ESB, granted). Also I find the Luke/Vader/Emperor scenes in ROTJ to be the darkest bits in the whole OT, so my question stands, why is Empire considered to be the darkest in the Original Trilogy? I think all three have their moments, and that's what makes these films what they are.