Also consider the psychological effect it has on the enemy to know that it doesn't matter how many Marines you've killed... they keep advancing and screaming bloody murder along the way. The Marine nickname Devil Dog came from Germans during World War II. The Germans had a strong defensive position with better numbers and eventually retreated after experiencing this exact thing.
It was WW1. The name came from a mispronunciation of the german words, "teufel" and "hunde" which mean devil and dogs. The mispronunciation though comes as "teufelhunden" which is incorrect. Its believed that an American came up with it after hearing a German say it about the Marines sometime before or after the the battle of Bealleu Wood.