Many times in literature, light symbolizes good and dark symbolizes evil. This is a clue that Mr. McIntyre might not be the helpful attorney that people think he is. But it's important to note that he "prefers" the darkness, so perhaps there's some light inside him after all.
A man stood in the doorway, his face obscured by shadows, his suit as black as oil. (1.23)
Here's another instance of darkness symbolizing evil. The man in black doesn't seem to have any gray area. He's all black. He must wash his clothes in Woolite Dark.
"They just want [Grace's] fortune," Dan decided. (2.35)
Dan makes the assumption that every other Cahill in the family is an evil gold-digger. Their behavior at the funeral doesn't do anything to convince us otherwise. Is it possible that the entire family is evil? Or at the very least really, really greedy?
These clues will lead you to a secret, which, should you find it, will make you the most powerful, influential human beings on the planet. (2.107)
As we learned from Spider-man, with great power comes great responsibility. Will whoever finds the secret become evil and drunk with power? Even Amy and Dan? Or maybe they're just good enough to avoid that age-old trap.
Beware the Madrigals. (4.57)
Later, Irina Spasky and the Kabras talk about the Madrigals, too. Perhaps the Madrigals could be evil enough to unite the family against one common goal? Perhaps… not.
When [Ian] smiled, he looked evil enough to be an adult. (7.24)
It's easy to forget that many of the characters in the novel are barely teenagers. Ian is only fourteen. Is it possible for a fourteen-year-old to be evil or, as this sentence suggests, are only adults capable of evil?
The young she-devil had a tiny silver dart gun cupped in her hand. (7.44)
Ian's sister, Natalie, is also described as being evil by being called a "she devil." But remember, she's only eleven. If she's just copying what the rest of family considers appropriate behavior (i.e. lying, cheating, violence, and stealing), maybe we can't quite write her off as a total baddie.
"You know the family's got branches, right? Good Cahills. Bad Cahills." (11.55)
Jonah Wizard spells it out in black and white. There's good and there's evil, and the Cahills are pretty much stuck on one side without any other choice. This makes good and evil into a nature vs. nurture battle, and according to Jonah, nature wins. We bet Amy would beg to differ.
"More famous Lucians. [...] Not necessarily good or bad. But definitely a lot of powerful people." (12.46)
Here, Amy contradicts Jonah's earlier statement, by referring to some earlier Cahills as neither good nor evil. From this, we can deduce that Amy doesn't buy into the "people are purely good or evil" stuff that Jonah does. This is appropriate since Amy, and her chronic tendency to abandon family members in dangerous situation (see our character analysis if you don't know what we're talking about), isn't entirely good or evil herself.
"A fire. [...] Like Grace's mansion. Like what happened to our parents. We haven't changed in all these centuries. We're still trying to destroy each other." (17.59)
Evil seems to be an endless cycle that's hard to break. It seems that people more often "fall from Grace" (pun intended) than they go the other way. Does evil always win out in the end?