Authorities say 25-year-old Richard Giga attacked the staff member, identified as 61-year-old Leland Wood, inside the Renaissance Project on Ulster Heights Road in Ellenville just after 3 a.m.
Neighbors say the horrifying ordeal read like a horror movie, and that they can only imagine the terror the victims felt.
"You know when I heard helicopters early this morning, I knew something was going on, but I didn't know if it was an accident or something, so it was scary," said Debbie Brissett, an Ellenville resident.
Giga tried to leave the drug and alcohol rehabilitation center, but when he was stopped by Wood, he allegedly attacked and fatally stabbed him.
A 35-year-old nurse reportedly tried to help the security guard, but police say Giga kidnapped her and forced her to drive him from the scene.
About a mile away from the center, police say he stabbed Keri Reynolds several times in the abdomen and chest while she was driving, causing the car to flip and crash.
Reynolds was found by a passer-by, who rushed her to St. Francis Hospital in Poughkeepsie.
"I came down across a car accident, and the lady came out screaming for help, screaming for help that she had been stabbed," the Good Samaritan, Guy Churchill, said.
Reynolds is in the intensive care unit in critical condition, but is expected to survive.
Giga fled on foot, but was taken into custody shortly after 6 p.m. following an intense manhunt.
Police tracked him down after he made a phone call from a vacant home.
He had barricaded himself inside a two-story brick house on Ulster Heights Road near where the crash happened.
Police took him into custody without incident.
"We had concern this morning that the buses picking up the kids, so we canceled that," Ulster County Sheriff Paul VanBlarcum said. "And then obviously, the greatest concern would be that he would break into another residence, an occupied residence and that the case would grow even worse from there."
Schools are open Thursday morning.
Authorities say Giga pleaded guilty to grand larceny in 2008 and was required to enter the residential treatment center prior to sentencing after it was determined he had a drug and alcohol addiciton. He entered the center on January 8, 2009 and has not yet been sentenced for the conviction. He had no prior criminal record.
According to Renaissance's website, the center is a 100-bed facility in Ellenville, in the Catskills 75 miles north of New York City.