Lerner said Hiller chose to return to the sprawling plant on Roosevelt Boulevard at Byberry Road after being escorted out after an argument with three coworkers at 7 p.m.
Lerner said Hiller went to her car, got a .357 Magnum she legally owned and kept under the seat, and returned after threatening a guard.
"Miss Hiller formulated and carried out a premeditated intentional plan to shoot three people," Lerner said. "She intended to kill them. She succeeded in two cases."
Lerner set sentencing for Sept. 24, although it is a formality; first-degree murder carries a mandatory life prison term without parole.
Hiller, a short woman dressed in a black Muslim gown and head covering, did not testify and did not appear to react to the verdict.
Families of the victims declined to comment afterward.
Hiller agreed to the nonjury trial in a deal in which the District Attorney's Office withdrew plans to seek the death penalty if Hiller withdrew her plans for an insanity defense.
Victim Bryant Dalton, 41, testified about working with Hiller and the other victims as a mixer on the third floor of the plant where they made Oreo and Lorna Doone cookies, Ritz crackers, and other popular snacks.
Dalton said he left the men's room and was walking back to his mixing station when Hiller called him and Wilson over.
" 'I want you to be a witness,' " Dalton said Hiller told Wilson. Then, he said, she turned to him, saying, " 'I'm starting to get sick of all the games you are playing, spraying me with chemicals. I'm going to [expletive] you up.' "
Dalton said he was stunned and then angered at the threats, and they all wound up in a manager's office along with Brown and other colleagues before Hiller was escorted away.
"I had no idea what she was talking about," said Dalton.
A half-hour later, Dalton said, he, Wilson, and Brown were in the employee lounge when the door burst open and Hiller stood there with a gun, swearing at them for "costing me my job."
Dalton said he heard a shot and felt a sting as a bullet went through the right side of his neck, exited the left, and buried itself in his shoulder.
Dalton said that the bullet remained in his shoulder and that he returned to work only in February.
Defense attorney Constance Clarke, who represented Hiller with Wendy Ramos, argued that Hiller was motivated by the deeply held belief that for years her coworkers were poisoning her.
"It was without malice," Clarke told Lerner. "She killed in the unreasonable and mistaken belief that she needed to commit the act to protect herself."
Assistant District Attorney Gail Fairman, however, said Hiller was "the aggressor" from the start of the argument with Dalton and then became "the angel of death stalking these hallways."
Contact Joseph A. Slobodzian
at 215-854-2985, firstname.lastname@example.org, or @joeslobo on Twitter.