Erik Menéndez is outraged over the new Netflix series about how he and his brother Lyle notoriously murdered their parents in 1989 … and he says Ryan Murphy went out of his way to slander the infamous siblings!

Here’s the deal … “Monsters: The Lyle and Erik Menéndez Story” debuted Thursday on Netflix … dramatically chronicling what happened before, during and after the brothers shot their affluent parents José and Mary Louise “Kitty” Menéndez.

The actual Erik called BS on the series … and lambasted RM for his “naive and inaccurate” depiction of the brutal home slayings that made international headlines.

Erik straight-up accuses RM of going out of his way to paint the brothers … who, again, were convicted for brutally murdering their own parents … in the worst light possible … by leaving out the brothers’ allegations of severe abuse by their father.

“So now Murphy shapes his horrible narrative through vile and appalling character portrayals of Lyle and of me and disheartening slander,” Erik says.

As is pretty much common knowledge — the brothers were convicted of first-degree murder in 1996 and alleged in the trial that the double murderers were done out of fear for their lives after decades of violent and sexual abuse … and not to get their hands on the gigantic inheritance.

Remember, Menudo member Roy Rosselló alleged their father, Jose, raped him … and their attorney Mark Geragos told us last year why Roy’s claim — and a newly-found letter written by Erik — could make all the difference in court if they get a new trial.