We wonder what could ever drive the brothers Mario to such a dispute that would need to be resolved with firearms? We then remember the Mushroom Kingdom being home to a certain princess and find ourselves thinking that people have killed over less. But, come on! That's your brother, Mario, your blood!
All joking aside, this snapshot comes from Garry's mod where apparently you can play some Half-Life 2 as Mario or Luigi, and infringe on some copyright laws. Anyone interested in doing so probably needs to keep an eye on Garry's Mod here, which all the cats tell us is pretty much their only set of pajamas.