You should definitely remove the aggressive animal. Wood turtles develop kind of a hierarchy system, so you may notice one of the others step up and "take charge". The best thing for Woods is to give them a large enclosure with lots of structure. Breaking up lines of sight and giving little "hidey holes" really helps to calm things down.

The damaged animal will need to be quarantined until healed. If not, the cagemates will likely constantly bite at the wounded area.