I don’t get how double edged sword means something good and bad. The phrase implies that you’ll cut your opponent with one edge and cut yourself with the other, but I don’t think that’s how swords work. Granted, I’ve never used a sword, let alone, compared the qualities of single v. double edged ones, but I have seen a lot of movies.
I have a scar on the pointer finger of my left hand that I got from mismanaging a banana situation. I had two bananas hanging from a banana hook and I wanted to eat one, but somehow let the last one remain on the hanger. Now, if I simply pulled the bananas apart, neither one would have enough stem to hang on a hook. I decided that I would cut the stem off of one, that way there would be a little stem-hook so the other one could hang.
For some moronic reason, I did this with the bananas still on the hanger. My knife sliced through the stem and right into my finger. Side note: the saying, you never cut yourself with a sharp knife, is more about being responsible with your equipment, and properly maintaining it, than the actual ability of a knife to cut a human. This was a very sharp knife.
I have something good and something bad. Good things: one banana to eat, and one banana that can hang. Bad things: a deeply cut finger, and two bloody bananas. It would seem then, that my single blade knife has become a double edged sword. I consider this an upgrade. Now, I think I’ll go slay some dragon fruit.