Texts 5-6

ashastra-vihitam ghoram
tapyante ye tapo janah
dambhahankara-samyuktah
kama-raga-balanvitah

karsayantah sarira-stham
bhuta-gramam acetasah
mam caivantah sarira-stham
tan viddhy asura-niscayan

Translation

Those who undergo severe austerities and penances not recommended in the scriptures, performing them out of pride and egoism, who are impelled by lust and attachment, who are foolish and who torture the material elements of the body as well as the Supersoul dwelling within, are to be known as demons. 

Commentary by Srila Prabhupada 

There are persons who manufacture modes of austerity and penance which are not mentioned in the scriptural injunctions. For instance, fasting for some ulterior purpose, such as to promote a purely political end, is not mentioned in the scriptural directions. The scriptures recommend fasting for spiritual advancement, not for some political end or social purpose. Persons who take to such austerities are, according to Bhagavad-gita, certainly demoniac. Their acts are against the scriptural injunctions and are not beneficial for the people in general. Actually, they act out of pride, false ego, lust and attachment for material enjoyment. By such activities, not only is the combination of material elements of which the body is constructed disturbed, but also the Supreme Personality of Godhead Himself living within the body. 

Such unauthorized fasting or austerities for some political end are certainly very disturbing to others. They are not mentioned in the Vedic literature. A demoniac person may think that he can force his enemy or other parties to comply with his desire by this method, but sometimes one dies by such fasting. These acts are not approved by the Supreme Personality of Godhead, and He says that those who engage in them are demons. Such demonstrations are insults to the Supreme Personality of Godhead because they are enacted in disobedience to the Vedic scriptural injunctions. The word acetasah is significant in this connection. Persons of normal mental condition must obey the scriptural injunctions. Those who are not in such a position neglect and disobey the scriptures and manufacture their own way of austerities and penances. One should always remember the ultimate end of the demoniac people, as described in the previous chapter. 

The Lord forces them to take birth in the wombs of demoniac persons. Consequently they will live by demoniac principles life after life without knowing their relationship with the Supreme Personality of Godhead. If, however, such persons are fortunate enough to be guided by a spiritual master who can direct them to the path of Vedic wisdom, they can get out of this entanglement and ultimately achieve the supreme goal.

Commentary by Sri Visvanatha Cakravarti Thakur

"You asked the question 'What is the position of those who, giving up the rules of scripture but with faith, without enjoyment of objects, perform worship?' Now hear the answer." The Lord answers in two verses.

They perform austerities which are harmful to living entities (ghoram). The mention of austerities is representative of other acts such a japa and sacrifice as well. They perform all these acts against the rules of scriptures. The absence of desire for enjoyment and being filled with faith (sraddhanvitah) are understood in this context. They are endowed with ostentation (dambhah) and pride (ahamkarah) because without those two, they could not transgress the rules of scripture. They also have desire for eternal youth, eternal life, kingdom and such things (kama), attachment to their particular austerity (raga), and ability to perform the austerity (bala), like that Hiranyakasipu and others. Endowed with these qualities, they starve or cause suffering (karsyantah) to all elements such as earth (bhuta gramam), to me (mam) and to my amsa the jiva, situated in their bodies. Know these people to be also situated on the level of asuras.