WEBSTER, N.Y. — On Monday morning when darkness was still raw, William Spengler Jr. armed himself with a rifle, a revolver and a shotgun. He had killed before. Harboring a deep-seated hatred of his sister, who lived with him, and a desire to harm his neighbors on a beachfront strip off Lake Ontario, Mr. Spengler composed a rough, typewritten plan that foretold of the destruction to come.

“I still have to get ready to see how much of the neighborhood I can burn down and do what I like doing best — killing people,” Mr. Spengler, 62, wrote, in a note the police recovered.

It had been 32 years since he beat his grandmother to death with a hammer in the Lake Road house next to his.

As Christmas Eve dawned in this suburb of Rochester, local authorities say, Mr. Spengler set fire to a car, as a trap. When an engine company came roaring down the street, he started shooting at the first responders, most likely from his Bushmaster .223-caliber rifle. It was the same type of semiautomatic weapon used in the school shooting 10 days earlier in Newtown, Conn.