Showing courage isn't always associated with noble deeds. One can be both brave and foolhardy or brave and wicked. Peter Pettigrew demonstrated courage in cutting off his own finger to frame Sirius Black. And it took courage, even among Death Eaters, to serve Valdemort. Bravery is an adrenaline rush but it is the rational mind that directs the energy. The sorting hat may have detected the physical energy necessary for brave actions even though there was a distinct lack of judgment. So it is possible that Pettigrew could have been sorted into Gryffindor at the age of 11.