The synopsis in the manual also states that Bowser turned the residents of the Mushroom Kingdom into “stones, bricks, and field horse-hair plants.” In a given playthrough, most players probably smash a lot of bricks. Bricks which used to be Mushroom Kingdom people, who are now dead. Because Mario killed them.

It’s a big maybe on Mario being the hero because he may or may not actually succeed in reaching Bowser and rescuing the princess depending on how much the player happens to suck, and/or of Luigi winds up being the victor instead.