[Warning: Major spoilers ahead]
What you're saying isn't 100% accurate
I finished the game and saw screenshots of the other endings in the developer’s Discord group. You should know that the game actually conveys a simple moral: all your choices lead to different paths and consequences. If you got the worst ending, it’s because you yourself decided to make the worst choices at certain points, which led to that ending (the demon ending).
There are actually three different endings (from what I saw / I got ) : the Angel Ending, the Demon Ending, and a neutral ending without ending photo (?) which is fairly positive and, in my opinion, the most interesting and revealing.
The story is indeed generally depressing for the main character, but if you’ve consistently made good choices from the start—or if, toward the end, you make certain good choices—the game gives you the chance to save the main character, leading to the Angel Ending or the neutral ending, which I think is ultimately very positive since her life is saved.