(cross-post) If Christianity stemmed from Judaism, then why are doctrines such as Original Sin and Eternal Conscious Torment virtually non existent in the Hebrew Bible?

I’ve been wrestling with this question as of recent. Correct me if I’m wrong, but the common belief is that Christianity is essentially an offshoot of Judaism. If that’ the case, then why are some of the essential doctrines of Christianity virtually non existent in the Hebrew Bible?

For instance, both the Hebrew Bible( akak the Tanakh) and The Gospels/Epistles talk about sin, but the way they discuss this matter differs. In the Tanakh, it seems as if sin is more of a matter of personal responsibility, as opposed to a force that enslaves us without our consent, due to Adam’s disobedience. In fact, after Adam ate from the tree of the knowledge of Good and evil, you would think that God would use that time to specify that a consequence of their disobedience would be that all of mankind would be born into sin, but that’s never mentioned.

To expound on my previous point, the Hebrew Bible never seems to imply that man is completely incapable unable of freely choosing good over evil, life over death, unlike Christianity, which espouses that mankind is a slave to sin, completely unable to exercise dominion over our sin nature.

Also, if the consequences of sin would lead to eternal torment/darkness, etc., you would think such a thing would be outlined in the Tanakh, however such descriptions of the afterlife don’t really seem to exist. In fact, books like Ecclesiastes seem to imply that both the righteous and unrighteous will all go to Sheol.

I guess I’m just wondering if some of our Christian doctrines stem from outside sources, or one mans interpretation of scripture(Augestine for example).

Additional questions:

1. Does the holy book of Judaism teach us that mankind is a slave to sin?
2. Are Christians allowed to make questions in this subreddit about Judaism?
3. Has anyone from this subreddit converted from Christianity to Judaism?