Double-edged sword

A metaphor for something that has both favourable and unfavourable consequences dates from the 17th century. It is quite hard to understand the logic because the double-edged sword has been used for thousands of years as an extremely effective weapon that cuts both ways in the hands of an expert swordsman. One can only assume that in the hands of an incompetent swordsman it could conceivably inflict self-damage and hence give rise to the metaphor.