It seems you are having trouble understanding this Verse:
16.19-20 Those who are envious and mischievous, who are the lowest among men, are cast by Me into the ocean of material existence, into various demoniac species of life.
Attaining repeated birth amongst the species of demoniac life, such persons can never approach Me. Gradually they sink down to the most abominable type of existence.
It is only saying that until someone stops being envious of God they never approach Him. When they stop being envious, then they can approach Him. This is the Teaching of the Gita.
I would like to point out however that no one is born envious, nor does a consciousness of envy transfer over from a past life into a child who is in the womb. There are some self evident truths. Demons are not born into wombs. Cute little animals aren't demons nor are they subhuman, little baby tigers or puppies Love just as much as humans do, and that makes them just as intelligent, because real intelligence is that of the Jiva, measured by Love. Many Self-Realized souls are born into animal bodies, and it is said that Krishna descends into every species of life. Demons are generally un-renewed, un-born-again beings who after death go to a place of punishment, and demonic species of life in hell or in the lower realms are a continuation of their previous consciousness without a birth, it is without being sent into a womb of a mother. Being made new and being born again or for the first time makes you innocent, and that innocence can be Eternal, then if one becomes fallen they have to become twice-born, which is the standard ritual for Vaishnavisa initiation for everyone who enters the Brahma Sampradaya, even though there are many naturally twice-born beings. Just like Jesus Christ says "to enter the Kingdom of Heaven, you must be born again." Birth is a purifying thing.
For lack of a better word
demons are said
to be born in demonic species, but no one can be born a demon. After death the servants of Yamaraj may send a demon to another body in a process of transmigration, but this is not a birth. For example after one burns up in the Lake of Fire without repentance they can be sent to a Universe where their body is composed of a dark mound on an empty planet. These bodies are made by the creative impetus of the servants of Brahma of that Universe and of Yamaraj, and ultimately the Supreme Lord who presides over that Universe. The demons in these bodies are traumatized by their own sins and past, so they have little consciousness and hardly remember anything. I have meditated on that Brahma walking inbetween these mound-like bodies of these fallen beings and helping them with his mind, and there was overwhelming darkness. For many of them, in that Universe, it is a long road to recovery. That is what is meant by a demonic species of life. It is not a birth from a womb but a transmigration of the soul by another means to ensure peace and order in the material world, so that no one is born evil. There are no evil children.
I know that both the Vedic and Buddhist Scriptures use the term that demons are born into hell, but since things have been bent in the philosophy for so long it is difficult to provide this type of explanation for such a long going misunderstanding every time it comes up, so even those who are fully aware of these things, just like the Buddha and Krishna are and were, still use ancient terminology and such traditional terms, although they are not accurate. It is possible to be born into a hellish condition or even on a low planet for a human being or animal, but these are not demons, just unfortunate innocents. Children who come from wombs are always pure and innocent.