"Hate not the sinner"
Those words were uttered by Sister Christabella to the young mother Dahlia Gillespie, (played by Deborah Kara Unger) as she went to talk to the religious leader in concern for her daughter. This is seen as a flickering flash back, burned into the memory of Alessa, as she recounts her tale to another young mother by the name of Rose Da Silva who is frantically searching for her daughter, Sharon.
Both the daughter and mother were having trouble fitting into the community not because of Alessa's lack of a father, but rather Dahlia laying with a man while being unwedded to him.
Many traditional Christians would call it adultery.
To the town of Silent Hill it was called, 'Lust', and it was a Sin.
It was made clear that Alessa was Dahlia's sin of lust and was a Witch in the eyes of the woman's cult. Even the children tortured Dahlia's only child on the encouragement of parents and teachers since Dahlia refused to 'repent'. Dahlia, no matter how misguided and sinful in eyes of her faith, loved her daughter and tried to stand up for her only child.
It is seen in the gentle memory of the beautiful Dahlia comforting Alessa as she wept in the bathrooms of her school and when she confronts the cult leader, Christabella.
No mother is perfect and social death truly hard to bear when one is a single mother, deep in sin, in a town controlled by faith. Dahlia, in a moment of weakness, gave into the illusion she and her daughter could be purified, and thus saved. As she exited the car and walked down the hall behind her precious child, the facade began to fade and it was much too late.
All she could do is stand there and utter, "What have I done?" before being blocked by the cult as Alessa screams for her.
There are good people in the world and want to help people, but by the time Dahlia found them and got their assistance... The Cult rite had failed and Alessa was badly burned, Dahlia crushed by the true sin she had committed. No words of comfort nor any amount of love could save her daughter from the Hell Dahlia sent her into- unwilling or not. Dahlia was forever broken at that moment.
"In the eyes of a child, mother is god."
In the present time of the movie, Dahlia continues to be spat upon and take the harassment of the Cult as she screams for the reaping of sins they had committed against her child. Dahlia no longer cared on her treatment or looks, her sole goal was to make the ones who had hurt her child pay for their crimes. I believe she wanted this in the name of justice for her child and atonement for her sins.
As for Dahlia's ending:
The woman lives, after Alessa unleashes her justice onto the Cult, and she ponders why her daughter spared her life. Rose merely looks at her and says, "In the eyes of a child, mother is god."
I believe Alessa did love her mother, but letting Dahlia live was a harsher punishment then death could give her.