But those who put much weight on sin, see people as generally bad and dangerous until proven otherwise. I will use "Christian" as a stand-in for that position throughout this OP.
It's not that people are bad and dangerous, but the recognition that people are far from innately virtuous.
The Humanists idea of man is in opposition to that dogma. A Humanist sees people as generally good and trustworthy until proven otherwise (and even then as having an innate dignity).
The idea that human beings have innate dignity is itself a development of Christian doctrine. Do you think Aristotle and Plato, who believed slavery to be an institution of natural law, had any concept of innate human dignity? For pre-Christian antiquity, the bulk of humanity exists only to benefit those who by nature are fit to command.
The idea that people have innate dignity is far from some self evident truth that only a Christian would be perverse enough to deny. It is the exact opposite. The humanist is in debt to Christian doctrine here.
Another facet of the
idea of man in Christian vs. Humanist vision is the power and responsibility of people. (Thanks to
@Harel13 for reminding of this and finally motivating me to write this OP.) In the Christian diction man is powerless against the will of god and nearly powerless against temptation. In the Humanists mind man is responsible and capable to forge his own fate.
The moral law would be null and void if we were powerless against temptation.
There is not a single person to whom God denies sufficient grace to become righteous in His eyes. Yes, the virtue which God commands is tough and arduous but we are assured of divine help if we but fully trust in the divine will. We thus have a deep responsibility for our lives because this is the one and only life where we have the opportunity to acquire merit by the virtues. (Merit which sticks with us for eternity).
I'm less interested in debating which position is more right but more interested in debating which position is more useful.
Isn't it more psychologically sane to tell people and especially children that they have power and responsibility and to assume the same about others?
The trouble is that your understanding of what Christianity claims about human nature is a caricature. The doctrine of total depravity (the doctrine that humans beings are are utterly incapable of good) is not a "Christian" doctrine. It is a Protestant doctrine based upon sixteenth century interpretations of Saint Paul.