Foster the People played two sold-out shows at 9:30 Club on Monday, propelled by their mega-hit “Pumped Up Kicks.” (All photos by Kyle Gustafson/FTWP)

That song is “Pumped Up Kicks,” a bouncy hit that assumes the viewpoint of a guy who’s apparently about to perpetrate a mass murder. “You better run, better run, outrun my gun,” the audience sang cheerily when the tune finally arrived at the end of the band’s earlier set (the first of two sold-out Monday shows). Turning the homicidal sing-along into even more of a party, Foster and the people (the group’s original name) interjected a dance-remix passage, sampling and looping the chorus into a clipped, house-style groove. And the members of Reptar, the somewhat trippy opening act, rushed onto the stage, banging handheld percussion instruments. This spun the well-planned set back to its opening number, “Houdini,” which began with most of the musicians banging or clanging on something.