Kyle revealed the beef in Wednesday's episode of "The Real Housewives of Beverly Hills" ... she was asked what's appropriate for husbands when it comes to online interaction with other women.

KR was triggered, referencing "murder" ... then admitting she's fought Mo before over stuff like this.

In a separate interview, Kyle claimed Mauricio's DMs are filled with women who, according to her, don't care he's married and are always "the aggressor."

It sounds like these spats went down before their separation last July ... although she never directly addresses when the fight went down.

As we reported, Mauricio was having fun on the Aspen slopes last month with Lele Pons and Anitta -- he filmed the girls nearly naked skiing down a hill, and was later spotted dancing shirtless at a nearby restaurant.