'Mob Wives' Natalie Guercio -- Boyfriend's Gonna Sue ... Nightclub Let My Attacker Escape

The boyfriend of "Mob Wives" star Natalie Guercio wants to know how a guy with a boxcutter could slash him in a crowded nightclub, and then walk away clean ... and he's ready to sue to get answers.

London Rene tells TMZ he's already talked to his lawyer about taking Output Nightclub to court -- because he has two big problems with the Brooklyn hot spot's security team.

His first issue -- how alleged attacker, Rudy Lopez, got into Output armed with a weapon like a boxcutter. London says his attorney's requested security cam footage for a closer look at what happened when Lopez entered -- but he claims the club doesn't typically frisk guests.

Secondly, Rene says he subdued Lopez after the stabbing, took him to club security guards and asked them to hold him until police arrived. He claims security simply let him go.

We contacted Output ... so far, no word back.