When Agamemnon sacrificed his daughter, Iphigenia, the big guy became an official member of this group. Even though they all had their reasons—revenge, madness, the orders of the gods—nobody in this club wears a members only jacket.
High on everybody's list of worst mothers ever, Medea is most famous for slaughtering her two sons after her husband, Jason, ditched her for a pretty, young princess. Sure, she totally had a right to be mad. After all, Jason never would have gotten famous by bringing back the Golden Fleece if it weren't for some pretty slick sorcery on Medea's part. Still, killing your own children for revenge? Not a great call.
Famous for being the biggest, strongest hero ever to stomp the earth, Heracles also had an unfortunate incident of kid-killing. Though he spent most of his time slaying monsters, the hero of heroes also slayed his own children after being driven crazy by Hera. To atone for his crime, Heracles set out on his Twelve Labors, all of which were assigned by Hera via the weakling King Eurystheus. In the end, the joke was on Hera, though. Heracles' labors totally put him on the map as a hero, and eventually he became a god and married Hera's own daughter.
Abraham is a major figure in—count 'em—three big-time world religions. Judaism, Islam, and Christianity all think of him as one of their founding fathers from back in the day. Like Agamemnon, Abraham was asked to sacrifice his child by a higher power. When his God told him to offer his favorite son, Isaac, as a burnt offering, Abraham set out to do as he was told. Right before he did it, though, an angel swooped down and told him it was just a test.