He's decided that because the facts seem to add up a certain way, to not test them.
We have a passionate but troubled person, who is upset about her schoolmates bullying her and believes rightly or wrongly that her parents are killed. Who put her life at risk to make sure Crest was on her side.
Next we have being with no body, and a freaking flesh eating summon, who for all Crest knows, could have hired actors to convince Crest. We know NOTHING about this person. Who actually admitted to killing the parents earlier, meaning at least one of his statements is a blatant lie. And yet Crest trusts them instead of someone he's spent time with.
This is actually the second time Crest has let HIS feelings of betrayal get in the way of trust. The first was when Maytag came out as an emotionless girl. Rather than being "we love you anyway" Crest acts like she did something horribly wrong.
Dude WTF?!?
Now, in terms of feeling betrayed. I can get that. I can even get the feeling of "what's the point?" futility. But I think when a person has shown trustworthiness, it seems like you should at least check them. A person known for killing people (to the point of a patented spell), on the other hand, is likely a murderer.
I pride myself at having some knowledge of people's character and of logic. Logic tells me Qtalda is lying about something. Knowledge of Suspiria tells me she's a revenge obsessed person, not a manipulator. Without a reason for revenge, her past makes no sense.
bulmabriefs114, her memories could be altered in a controlled or uncontrolled manner, which is why there's even a reason Crest can entertain the side of the story Qtalda and friends gave him.
Ohhh, I misread. Qtalda's story. Yes, I suppose that would make some sense. Still, it would not render her guilty of anything, just dangerous by reason of insanity.
In either case, if Crest likes Suspiria, there is no reason to go after her on Qtalda's behalf. If he doesn't like her anymore, there is also no reason to go after her. This is a Morton's Fork.