Why was Dahlia such a doormat,
According to the actress, Dahlia was weak and blind to the truth, so she didn't speak up in time. "Brainwashed" would be a good word for it. Considering she and Chnristabella were sisters and the cult had been around for awhile, just imagine their upbringing. Their parents were likely cultists too and taught their daughters to obey the cult at all costs. Dahlia's weak personality didn't help either.
why did the Order consider Sharon/Alessa so much more evil than anyone or anything else,
The cult in the film is not The Order. They are a separate cult who stands in direct opposition to The Order. Gans hinted in an interview that The Order still exists in the movie timeline by saying Silent Hill has "many cults". Another hint was given when he stated that the woman in the hotel painting is Jennifer Carroll. There are a few reasons they would have hated Alessa more than any other. For starters, there was the whole lack of a father thing, and Dahlia's continued silence about it (which suggests she may very well have had an affair with a married man, or been raped). Then there's the history of witchcraft that is hinted as running in their family (which I think means Alessa's powers are probably some type of hereditary mutation). The woman in the church painting was specifically painted to look like a relative of Dahlia and Christabella, meaning that there could well be a belief that witchcraft runs in the family, an image Christabella would very much want to stomp out. Naturally, this would make Alessa's illegitimate birth even more suspicious to the cultists. By declaring Alessa a witch, Christabella reminds everyone that she is
not a weakling who will put up with any more witches in her own family line, thus immediately stopping any chance the cultists may cast suspicion on her for not considering the idea Alessa may be one of "family witches". The last nail in the coffin is probably the fact that Alessa very likely was showing powers long before the burning. If I recall correctly, a scene was shot that depicted Alessa causing butterflies to flock too her in an unnatural manner, causing her teachers and classmates to take notice of her strange abilities. In the original script, it's made clear that Alessa's powers were what broke the chain while she was being burned, though the final movie only implies it. Either way, those powers alone would have led to her being declared a witch. I believe the butterfly scene was likely dropped when they switched over to Dark Alessa's narrative in post due to the fact that Gans didn't want the audience to think Alessa was aware of her powers. According to Dark Alessa, anyone who is in as much pain as Alessa can create alternate realities and monsters, prematurely wilt flowers, and create doubles of themselves. Of course, this is wrong, but Alessa doesn't understand that. Gans portrayed her as genuinely believing she was targeted for not having a father (she wouldn't have understood the family history at that age), because she never understood and still doesn't understand her powers aren't normal. It confused the audience and was a terrible decision, IMO.
why did Christabella enjoy her work so much, etc.
That probably goes back to the whole family history thing. Christabella wants to put herself and her family in a good light, so she uses her chnarismatic personality to rise to power and ensure she would never be looked down on for her family history. By the time she burns Cybil, she's spent 30 years being attacked by supernatural forces, so when she finds a sacrificial victim who may actually help stop Alessa (in her mind), she's glad to do it and to end the nightmare she's been living. Sharon's burning would be an even greater example of this, considering she's one of Alessa's doubles.
However, I don't agree with jdnation's idea that Dahlia was the real bad guy behind this. The way they built up her character throughout the movie was as more of a crazed, harmless individual as opposed to the crazed psychopathic zealot from the games. I can't say that she was the bad guy--she didn't seem to have it in her.
I completely agree. As Deborah Kara Unger said, it was Dahlia's
silence that caused harm. Dahlia herself was weak, easily led, and certainly an unfit mother... but she was not evil or malicious.
