Does anyone know why black widow is a hero?

Because she's got mad fighting/gun skills, and is quick, like a Black Widow. :)

Born circa 1928, Natalia Romanova was apparently orphaned as a child when she was trapped in a burning building during an early attack on Stalingrad by enemy forces. Ivan Petrovitch Bezukhov, a Soviet soldier, found Natasha in the inferno and rescued her. Although Ivan kept a close eye on Natasha as she grew, by the late 1930s she had attracted the attention of Soviet intelligence, which began her training. In 1941, she was almost brainwashed into serving the ninja clan the Hand, but was saved by Ivan, Logan (James Howlett, later Wolverine) and Captain America. Following World War II, Natasha was recruited to become part of the Black Widow Program, a team of elite female sleeper agents. Among the Widow’s instructors was the Winter Soldier, formerly Captain America’s sidekick Bucky (James Barnes); Natasha studied as a ballerina to cover for her true occupation. The Soviet state eventually arranged for Natasha to marry Alexi Shostakov, a champion test pilot. After a few years of a happy marriage, the KGB faked Shostakov’s death in a rocket test; grief for Shostakov drove Natasha’s resolve deeper and she continued her education with the Red Room Academy, finally being appointed the title of the Black Widow.

The answer is !