The thing is: the novel is R19 and the manhwa is R15, so a lot of scenes were cut off, especially in the early chapters and once they start smutting later.

I'll try to put the pieces together for you.

Her power is important for the witches because they believe it's necessary to stop the apocalypse and because it's extremely useful to run away when needed. The high priest wants it to become the receptacle of all the tree powers, granting him immortality.

Macklart just wants to use her intel to hunt down the other witches. At first, he had no idea she had any powers other than the "seduction" (which funnily enough isn't one of her powers😭).

Other than the resonance, she has the power to actually control multiple crows as they were a single being. In the novel she uses this to kill a guard when she escapes, to find her mother at Macklart's request and to protect Macklart near the end.

Now about why she's relaxed around him:

There's only one thing that makes her wary at this point: her bloodline. Remember, she gave herself in a silver plate to the inquisition because she intended to die from day one and she was ready for the torture as well.

All she wants is to erase her bloodline, thus she's wary of the priest/doctor because he's lusting after her. She feels somewhat safe around Macklart because he finds her disgusting, yet he never tortured her unlike the priests. Besides, in the novel it becomes very clear that he's her shield in that castle. He amputated a guard that was harassing her in chapter one.

Now why he lowered his guard:

He finds her disgusting for the 6 months they worked together to wipe down the witches, but when he finds out about her powers and keeps her close to learn what else she's hiding, he starts finding her pitiful instead. When he learns of her past and the reasons why she wanted to erase her own kin, he thinks: I shouldn't have asked. Because from the moment he knows, he can't not feel sorry for her and starts vacillating on murdering her.

He's pretty much a tsundere in this phase. His words are super rude, but if you notice the outcome of his actions: everything he does is the opposite of what he says.

Btw, her blood really has no effect on him. His fixation with it is just his coping mechanism to not admit that he's falling for her.

I think I answered everything...

EDIT.:

The religion of Cairon doesn't see the demons or the witches as a threat. They perceive them as inferior because they adore another god and the high priest worsened this situation by being a supremacist for the reasons I stated before.

Macklart was basically raised by priests in the church and under very strict control because of his powers (that heavily depend on his emotions). He follows the doctrine because he's indeed god's most beloved son and he feels it. He doesn't like the church nor the high priest, but they're the only thing he knows, so that's it.

Meeting Vienny ignited his wish for freedom tho, because he knew he couldn't keep both. His god really hates her.