The defense will probably be "I didn't think it'd kill.  I was just out for fun, scaring those kids, that's all"  Never mind that there's probably tons of warning on the package and gun itself: "This is NOT a toy", etc.  To be honest, I wouldn't think an airgun would penetrate clothing, much less kill, but I wouldn't shoot somebody with it, either, even in jest.

Several years ago a bunch of high school punks were out with paintball guns in middle of the night shooting parked cars, pedestrians and bike riders.  They were stupid enough to video tape their escapade, which made wonderful evidence when they were caught.