Straw Dogs

David Sumner, an American mathematician, comes to live with his glamorous young wife, Amy, in her hometown, a small village in a remote part of Cornwall, UK.
Amy's return is of particular interest to her ex-boyfriend, Charlie Venner, and his cronies, Norman Scutt, Chris Cawsey and Phil Riddaway, who are immediately resentful of the outsider who has married one of their own. David hires the men to carry out repairs to the isolated farmhouse he and Amy have rented, Trenchers Farm. Tensions in the Sumners' marriage soon become apparentâexplicitly so when Amy stands topless in a window in full view of the workmen.
When Amy discovers their dead cat hanging by a light chain in their bedroom closet, she claims the workmen are responsible. She presses David to confront them, but he refuses. Later, the men invite David to go hunting in the woods with them. During the hunting trip, the workmen take him to a remote forest meadow and leave him there with the promise of driving the birds towards him. Having ditched David, Charlie Venner returns to the couple's farmhouse, where he initiates sex with Amy. She at first resists but eventually appears to submit, repeatedly embracing and kissing him. As Amy and Charlie lie together, Norman Scutt enters silently and forces Venner at gunpoint to hold Amy down while he rapes her in a sequence far less ambiguous as Amy screams and struggles to break free, to no avail.
The next day, David, who is seemingly unaware of his wife's ordeal, fires the workmen. Later that week, the Sumners attend a church social where Amy becomes distraught after seeing the men who raped her. They leave the social early, and, while driving home through thick fog, accidentally hit the mentally handicapped Henry Niles, a local villager. They take Henry to their home. David phones the local pub to explain about the accident. However, earlier that evening Niles had accidentally strangled a flirtatious young girl from the village, Janice Hedden. Her father, the town drunkard, Tom, and the workmen looking for him, are alerted by the phone call to Niles's whereabouts.
Soon the drunken locals, including Amy's rapists, are pounding on the door of the Sumners' home. The local magistrate, Major Scott, arrives to deal with the situation, but is accidentally shot dead by Tom. David realises that he, Amy and Niles are now in mortal danger, and prepares to defend his household.
David throws boiling cooking oil onto the locals, stunning them temporarily. He then turns some music on loudly so as to make his footsteps inaudible. Tom enters the house, armed with his shotgun, but David forces the gun down with the fire poker, making Tom accidentally blow his own foot off. As he goes to turn the lights on, David spots Chris Cawsey in the living room. After a brief but tense standoff, David manages to beat Cawsey to death with the fire poker. At this point, the music has ended, and Venner approaches with a shotgun. Before he can shoot David, they hear Amy screaming for help. They investigate and find Norman attempting to rape her again. Realising Venner intends to kill him, Norman begs to be killed; Venner complies and shoots his best friend dead. Immediately after, David wrestles the gun out of Venner's hand and manages to kill his wife's rapist by ensnaring Venner's head in a bear trap. As David and Amy relax, Phil Riddaway appears and attempts to kill David, only to be shot dead by Amy. With the mob taken care of, David drives Niles back to town.

Question: What did David throw onto the locals?
Answer: boiling cooking oil