Monster or Monstrare in Latin implied a portent, a prediction or a demonstration of the power of Deities, namely Nature. This view was propagated by "the father"of Biology, Aristotle until it was replaced with the invasion by Barbarians of Western Europe who destroyed Greco-Roman civilizations and intruduced naive credulity in “super-natural” explanations of Biology, Medicine and Sciences in general – Barbarians were among the first large population to become Christianized. The newer central dogma became “Man rules Nature” on behalf of Jehova (now in vogue by corporate policies regarding pollution). The malformed became “monsters” in the “modern” sense and replaced notions inherent in “teratos” as Natural portents alluded to above. Monsters became products of “evil” - the result of women's “relations with d-evils”. Even in the 1900’s, when Human Embryology, Physiology and other basic medical sciences were already well established as a basis for Scientific Medicine, pioneers investigating malformations included in their books extensive discussions on “maternal impressions” generally known as the “evil eye” as being a potential teratogen. (see “The Manual of Antenatal pathology and Hygiene, Vol. 2, by J. W. Ballantyne, 1902)
I am surprised how little linguistic and mythologic evidence there is of sympathy toward the malformed. Perhaps this also translates into a thoughtless acceptance of "euthanasia" applied to the killing of the malformed as is being malformed justifies their killing (including by "interruption of pregnancies" following the prenatal detection of "birth defects".
Regarding "mercy" and the malformed, one exception stands out – Xolotl. "He" was one of conjoined twins, arguably the earliest lethal malformation arising virtually immediately after conception. His co-twin, Quetzalcoatl, also is a major deity often alternatively represented as a “feathered serpent”. Both were venerated by the Aztecs and populations in Meso-America. Xolotl, among many other domains, represented the malformed, human or otherwise - Quetzalcoatl stood for learning and knowledge. Their mother was the virgin Coatlicue. Xolotl also stood for death by lightning, a method favored by Zeus.
There is some homology of Xolotl with the twins Hypnos - Thanatos. Hypnos was renamed by the Romans as Somnus. Both appellations, obviously stand for “he who induces temporary sleep”. Thanatos stands for “he who puts to sleep” permanently. The realization by Teratologists that most mammalian malformations are lethal, led to the synthesis of the notion Teratothanasia to denote the spontaneous prenatal demise of most of the malformed. Man "Puting to sleep" animals has for a long time a euphemism for killing them, an act often presented as "euthanasia" as if ending life is an optimal choice. Perhaps then, suicide of adults with mental capacity to determine their own fate could be legalized and some precedents of this sort already exist. However,"euthansia of chidlren", malformed or not, is not and inherent right of parents to make. Infanticide is abhorent under any circumstances but ... precedents are already emerging - in The Netherlands, arguments that some children are "better off dead than alive" have won the legalization of what some my call "euthanasia" of the malformed. (see Groningen protocols concerning spina bifida and the ending of life).