dear krishnan, Brahmarakshasa originated from a Brahmana.. The genesis could have been in a number of ways.. many brahmins become really learned in vedas and saastras but during lifetime do not observe the nityakarma anushtanams or engage themselves properly in karmakaanda. Going to temples or namasankeertanam would not be of much avail. Brahmin's duties have to be scrupulously followed. There are avaricious brahmins well versed and could perform yagas yajnas and paurohitya well. But they find wrong means to cut short the procedure if they don't get the huge dakshina they expect. There is another category who, though having knowledge, lack in aachara. When observing deeksha for vedic karmas the ritwig, hota etc., are expected to maintain a strict regime of mental and physical purity and also continence in food and abstaining from sex etc.,. Such restrictions are given a go by. Then there are brahmins, though learned, who commit various paatakaas(sins) and do not expiate for them during their lifetimes. When such and similar brahmins die they become pretaatmas,denied permission to enter pitruloka, and form a group wandering in this earth as braharakshasas. In fact we call brahmins showing such tendencies during their lifetimes itself as brahmarakshasas in jest.
For brahmarakshasas their vasanas and knowledge in vedas do not disappear with death. Usually they try to occupy some trees just in the periphery of the agrahaarams they infested during their lifetime. Now struggling with thirst and hunger with hardly anyone to pour til or water and pinda for them and even if it such things are offered being incapable of partaking in such offer, they sit restless, sleepless and mad perched on the trees. Whenever a brahmin passes by all alone especially after sunset, this rakshasa will come down and start asking him questions on vedic and scriptural subjects and if the unfortunate pedestrian is not able to answer, the angry brahmarakshasa will give the former a thorough thrashing (but they do not kill intentionally..). and the victim could be found lying unconscious under the tree in the morning.
As you know, such brahmins we see aplenty during their lifetimes and their tribe is ever on the increase. So whenever a sacred Karma or kumbabishekam etc., is conducted, and the devaprasna or ashtamangalya prasna are taken up, invariably the presence of some Brahmarakshas is found and pariharams are prescribed. Praayaschitthams are offered by the community on their behalf and the mollified brahmarakshasas are invoked into some silas (stones) or the wood from the gooseberry tree and given a place in a group in the outer precincts of the temple and offered daily pujas etc. It is believed that such expiated souls will transform themselves into Vishnuroopam after due penance.
There is a scientific explanation for people falling unconscious under a tree during nighttime. During the sunny day, there is a lot of photosynthesis, the leaves absorbing carbon dioxide and giving out oxygen, but the process stops with the sunset. Then the leaves and other parts of the tree have to breathe and their bottoms are rarefied, with percentage of oxygen pathetically low. So anybody landing up walking along a row of trees or plants can feel the lack of oxygen, become breathless, hallucinate and swoon.
Anyway brahmarakshasas have become interesting characters in local village lore. In Tamilnadu the attack by Muni is similar to this phenomenon.
There is one school of opinion that some of the brahmarakshas are being worshipped as kuttichathans in some places and many dirty tricks are played by people through their good offices.
The brahmarakshasas are notorious for their love for intoxicating drinks like patta (hooch), and better commodities. Maybe this is inherited from their former habits in their lifetimes.