A Long Island illegal migrant allegedly fatally stabbed his female neighbor at least 50 times with knives, then lay in wait for a Wendy’s coworker before slashing her to death, too, new details of the grisly slayings reveal.

The shocking account of the vicious Nassau County murders surfaced as Rony Yahir Alvarenga Rivera, a 22-year-old from El Salvador, was arraigned on first- and second-degree murder charges over the weekend — with a motive for the attacks still publicly a mystery.

“I’ll sum it up in one word — anger,” police homicide Chief George Darienzo said after the arrest without elaborating.

Rivera faced a judge for the first time Saturday just hours after his arrest.

“He waived Miranda [rights] and gave a full confession,” Nassau County prosecutor Alex De Palo told Judge Sean Wright at the arraignment. “The police observed on the defendant bloody clothing and cuts on his hands consistent with using a knife.”

The migrant, who entered the US illegally in 2016 as an unaccompanied 12-year-old and was allowed to remain in the country under federal policy at the time, was ordered held without bail.

Rivera was scheduled to return to court Tuesday for a routine hearing, but his lawyer waived his appearance before the judge, with Rivera now due back on until May 19.

The bloodshed allegedly began around 9 p.m April 30, when the migrant stabbed his upstairs neighbor, 32-year-old Eddy Raquel Hernandez Castillo, multiple times inside the Valley Stream rooming house where they both lived, police said.

De Palo said she was stabbed “upwards of 50 times,” with her body left “in a pool of blood.”

Rivera then allegedly hopped on a train and went to his job at a Wendy’s in Island Park, where he lay in wait by the dumpster outside for coworker Ana Maria de Aguila Cordova, a 42-year-old mother of two, for about 20 minutes.

“When that coworker took out the trash, he then attacked her, again without justification, using those same knifes, stabbing her upwards of 25 times and leaving her to die,” De Palo said.

Several hours later, the accused killer called cops from a 7-Eleven in Lynbrook and allegedly confessed his crimes to the responding officer.

He was booked and charged with murder.

“This is just an example of what happens when you have open borders,” Nassau County Executive and GOP gubernatorial candidate Bruce Blakeman told reporters after Rivera’s arrest.

“Why are we letting an unaccompanied minor into our country to run around, basically without any supervision whatsoever?” Blakeman said. “And then that individual grows up and automatically gets legal status in our country.

“This is a policy today that resulted in two women getting killed.”