In "The Birds" by Daphne du Maurier, what are some details that suggest that an evil force might be directing the birds to turn on people?

We experience the story from the point of view of Nat, who is intimately aware of the normal rhythms of nature in his area. As we watch what is going on with him, we come to share his deep sense foreboding that the birds are behaving in unnatural ways. We see as he does that the gulls are massing, then that they are heading toward the farm, details which make it sound as if the birds are especially targeting people: 

They were coming in now toward the farm, circling lower in the sky. The farm, then, was their target.

When they attack Nat, it also seems clear that they want to harm people in particular: 

They kept coming at him from the air—noiseless, silent, save for the beating wings. The terrible, fluttering wings. He could feel the blood on his hands, his wrists, upon his neck. . . . With each dive, with each attack, they became bolder. And they had no thought for themselves. When they dived low and missed, they crashed, bruised and broken, on the ground.

The fact that the birds have no thought for themselves is especially ominous: most animals have an innate survival instinct. This detail suggests that they do not care how much damage they suffer as long as they inflict suffering on humans.

The birds also target Nat's neighbors, the Triggs, and have the intelligence to rip the phone from the wall:

Trigg’s body was close to the telephone. He must have been trying to get through to the exchange when the birds got him. The receiver was off the hook, and the instrument was torn from the wall.

At the end of the story, Nat apparently accepts that it is all over for his family as the hawks concentrate their attention on breaking down the door. The hawks are engaging in a planned and calculated action, not a random or inchoate attack:

Nat listened to the tearing sound of splintering wood, and wondered how many million years of memory were stored in those little brains, behind the stabbing beaks, the piercing eyes, now giving them this instinct to destroy mankind with all the deft precision of machines.

The final quote suggests that the evil lurks inside the brains of the birds themselves.