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.

How The Post told the story of the horrific Long Island murders.

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.…