To be honest, Morathi and Malekith's background is among the best - Caledor Dragontamer's is annoying in that much like Teclis, so much of his victories are Deus Ex Machina - for example in the Son's of Ellyrion book, Teclis gets nearly killed by the equivalent of a Warhammer Fantasy Suicide Vest wearing elf, and is incapacitated, yet suddenly gets healed by a magical influx then manages to single handedly wipe out the Dark Elf Invasion, he also managed to single handedly wipe out Be'lakor's army, he single handedly routed the Dark Elves on the Finuval Plain, he single handedly destroyed a Greater Daemon with a single spell after his Brother was poisoned etc).
However, Morathi was never plain evil after her rescue - she was simply proud, if manipulative, but all High Elves were/are, and the fact that her son was turned over in favour of Bel'Shanaar for Phoenix King simply because of both father's actions damning one and empowering the other drove her insane. Malekith stayed strong, and soon began the purge of slaying the Cults of Pleasure, with the intention of finishing it with his mother's death. However, she was too strong, and the maternal bond between them gradually turned Malekith into what he became.
People blame Chaos for Driving Malekith insane - more likely, you could go as far back as drawing the Sword of Khaine has caused the Sundering, for should Malekith have been accepted by the Everqueen to become the Phoenix King, then most likely, the Sundering could have been avoided. It was just a chain of events, and hence the Sundering was the most visible effect of the curse that affected the entire Elven Race.