5. Baby Murder
For the most part, the Lannisters and their allies are a nasty bunch. As such, they are willing to do whatever it takes to keep one of their own on the throne after Robert's passing. And by 'whatever it takes', I really mean it. For instance, despite a royal decree naming Ned Stark the successor to the throne, the Lannisters emphatically get rid of him so as to ensure that Joffrey's ascension to power is uninhibited. Arguably as heinous as that betrayal is their method to secure Joffrey's - and more generally their own - long-term future as Ruler(s) of the Seven Kingdoms. It becomes known that Robert had several bastard sons to various women, each of whom would have a more legitimate claim to the Kingship than Joffrey and his line (since he isn't even Robert's son). So, the only thing to do is kill them all - and keep in mind that the former King's extramarital liaisons would have been spread out over a considerable number of years. We are treated to a scene where a baby boy is snatched from his mother's arms, before Janos Slynt takes a knife and commits a truly hideous murder.