If William McIntyre, Grace's attorney, were female, we'd call him a shady lady. Since he's not, we'll have to settle for the much less sinister sounding "man with a plan." Of course, we have no idea what his plan is. It might not be sinister after all. But when we consult our Magic 8 Ball, all signs point to yes. He's up to something. Something sketchy.
First there's his characterization. Early on we learn that "William preferred the darkness" (1.18). Who prefers darkness? Vampires, raccoons, albinos, and, in literature, evil people. It doesn't help William's case that he's fraternizing with the mysterious man in black, who always seems to be creepily watching the kids from afar when bad stuff goes down.
He also reminds Dan "of a vulture" (2.73), which gives us the unpleasant image of him circling for carrion to feed on. Dan later has a feeling that Mr. McIntyre wasn't always a lawyer, saying, "He'd seen things in his life—dangerous things" (9.28). What could he have been? An assassin, maybe? They love darkness, too.
Of course, there's always the possibility that all this sketchy behavior is misdirection. Seemingly saintly grandmother Grace didn't give the kids any tips either, so perhaps Mr. McIntyre is on their side after all. Where do you think his loyalties lie?