We might opionate our own ifs and buts and hows and whys, but here is a brief account of the origin of the rAkShasas from the purANas. (Ref: 'Puranic Encyclopedia' by VeTTam MaNi)
• A particular sect of asuras. The ancients had ordained that rAkShasas should not be killed at dusk. (vAlmIki rAmAyaNa, bAlakANDa, Canto 22, Verse 22).
• uttarAmAyaNa, contains the following story about the origin of rAkShasas.
‣ When BrahmA was reciting the Vedas at the beginning of KRtayuga he felt very hungry and certain forms emanated from his face. Those who were born from his anger assumed the form of rAkShasas and those from his hunger that of yakShas.
‣ The rAkShasas turned out to be evil folk killing and eating cows and brahmins. Praheti and Heti were the first born rAkShasas, the latter of whom wedded BhayA, daughter of KAla, and to them was born a son called Vidyutkesa. He married SAlakataNkA, daughter of SandhyA. Though a child was born to them they forsook it in the Himalayan slopes and went their own way.
‣ At that time Shiva and PArvatI came that way and after blessing the child returned to KailAsha. The child was named Sukesha. He married DevavatI, daughter of the Gandharva called Manimaya and three children were born to the couple, viz. MAlyavAn, SumAll and MAli. They did tapas to propitiate BrahmA and when he appeared requested him thus: "We must defeat Yakshas, Kinnaras, Gandharvas, Siddhas, VidyAdharas, Yama, Kubera, VAsava, NAga kings and daityas and dAnavas and we must not be defeated by any one. We must kill all enemies and they shall not kill us. We three shall never quarrel among ourselves."
‣ BrahmA granted all their prayers. The three, proud and haughty due to these boons, began roaming about consuming cows and brahmins. They asked VishvakarmA to build a city for them and he built for them LankA on Mount TrikUTa in the south sea.
‣ The three brothers took their abode in LankA. MAlyavAn, SumAll and MAli married respectively SundarI. KetumatI, and VasudhA, daughters of the Gandharva woman NarmadA. MAlyavAn begot of SundarI seven sons called VajramuShTi, VirUpAkSha, Durmukha, Suptaghna, Yajnakosha, Matta, and Unmatta, and also a daughter called NalA.
‣ To SumAll were born of KetumatI ten sons and also four daughters.
‣ Four sons were born to Mall of his wife VasudhA, and they became the ministers of VibhlShaNa.
‣ Then thousands of rAkShasas were born as sons, grandsons, brothers, nephews, etc. to the above and they lived in LankA, a terror to the whole world.
‣ While the daughters of SumAll, VekA, PuspotkaTA, KaikasI and KumbhinasI were once walking in the forest they saw Kubera on a visit to BrahmA in all pomp and glory. They understood that Kubera owed his pomp and glory to his being the son of Vishravas and therefore, the next day one of the four, KaikasI, went to Vishravas' ashram and prayed for children by him.
‣ Three sons called RAvaNa, VibhIShaNa and KumbhakarNa and a daughter called SUrpanakha were born to her by Vishravas. They secured boons by performing tapas and RAvaNa lived in LankA as king of the rAkShasas.