Meet your arch-villain. Seriously, this guy is practically Disney-level evil.
He Wants the Land
Granger's family used to own the plantation land that many of the local families, like the Averys, now sharecrop on. Worse, his family also used to own part of the land that now belongs to the Logans. He and a lot of the other white folks in town resent the Logans for having that independence, and Mr. Granger will stop at nothing to get this land back into his clutches.
Specifically, he gets one of his banker friends to cancel the Logan line of credit that gave them four more years to pay off their mortgage (10.137). Now, the total amount of the mortgage is due in full, or else the Logans lose their land. Uncle Hammer foils the plan by selling his Packard to raise the money, but it's a pretty good indication of the kind of man we're dealing with.
Worse (maybe), he's willing to turn a blind eye to murder. When the angry mob is about to hang T.J. at the end of the novel, Mr. Granger makes it clear that they can't do it on his land—but they're more than welcome to do it somewhere else. Like, even one step over the boundary line.
When Mr. Jamison appeals to Mr. Granger to stop the mob from hurting T.J., Mr. Granger "just stood there on his porch looking sleepy and bored" and tells the sheriff, "Hank, you take care of this. That's what folks elected you for" (12.129). So, he obviously doesn't care about this poor boy—remember, T.J. is the same age as Stacey—about to get murdered.
Ironically, Mr. Granger's greed ends up saving T.J.'s life: he stops the mob because he's too worried about his precious land burning. When he smells the smoke he leaps into action: "Mr. Granger comes running off the porch hollering like he's lost his mind. 'There's smoke coming from my forest yonder!' he yells. 'Dry as that timber is, a fire catch hold it won't stop burning for a week. Give that boy to Wade like he wants and get on up there!'" (12.130).
Well, at least we know where his priorities are.
A Bigger, Badder Boss Hogg
So, how does he get away with all of this? Harlan Granger basically has his own little justice system, complete with paid-off sheriff and other lackeys:
You know, back then the Grangers had one of the biggest plantations in the state and Spokane County practically belonged to them . . . and they thought it did too. They were consulted about everything concerning this area and they felt it was up to them to see that things worked smoothly, according to the law—a law basically for whites. Well, Harlan feels the same now as his grandmother did back then. (7.151)
Mr. Jamison here gives the Logans the deets about how Mr. Granger basically thinks the area is his to run as he sees fit. And we certainly see the sheriff taking his orders, as well as the Wallaces and others. He owns the land that the Wallace store is on, so has them firmly in his pocket. He is also influential enough to mount opposition against the Wallace store boycott (since part of the lost proceeds would affect his bottom line, too).
If this were a Disney movie, Granger would definitely get his comeuppance by the end. But he doesn't—which is a lot more realistic, and a lot more frustrating.