Balalaika isn't a villain; the only real villains you find in BL are the ones that try to kill Rock and Revy, for the most part...It's a series about grays no real hero-villain paradigms.

Well duh, technically she IS a villain due to her being a (way too ruthless) Mafia boss. In the context of Black Lagoon though she's more of a anti-heroine, much like the rest of the cast (which is also what makes the series so awesome, being made up with mostly immoral characters).