Conversely a scratch could turn deadly if it becomes infected, especially under adverse circumstances. These include poor wound care, an elderly, diabetic or immune-compromised individual or a virulent or antibiotic resistant pathogen. However, unless the scratch was self-evidently and undisputably the cause of death, even after the fact it would be unusual to refer to it as a 'mortal wound'.
Severe contusion leading to death is more commonly described as a fatal or mortal blow, which is also more widely used when one or both of the injury or death are metaphorical. While mortal wound is only strictly inappropriate when discussing a literal death not related to any penetrating or lacerating injury (by poison, for instance), it is simply uncommon in relation to failure or defeat. It would be equally appropriate and essentially synonymous to call the Great Depression a mortal blow or a mortal wound to Herbert Hoover's presidency and public career, although the former is more typical.