A blade sharpened along both long sides. There is no blunt side to rest a thumb on, and it cuts on the way out and on the way back. Every other use of double-edged can be read off that one picture: two edges means two directions, and one of those directions may be pointing at the hand holding it.
From the picture
同一把刀，砍出去也砍回來
把刀轉個角度就看懂了：你往前揮的時候，背面那一刃正對著自己的手。所以「好處」跟「傷害」不是兩件事湊在一起，而是同一把刀的兩個邊。
adjective · B2
Used in a double-edged sword (or a double-edged weapon) to say that one thing helps you and harms you at the same time, because the good part and the bad part come out of the very same thing.
雙面刃
一句話，兩個刃
刀的兩個刃也可以借來講一句話：表面那一面聽起來是稱讚，另一面在同一句話裡割了一下。
adjective · C1
A double-edged remark, comment or compliment carries two possible meanings at once, and the second one is not kind — it can be taken as praise and also as a quiet dig.
話中帶刺的
兩個方向同時出手
這一家只取「兩個刃＝兩個方向」，不取「其中一刃朝著自己」。所以整句話可以完全是正面的。
adjective · C1
Made of two parts that push in two different directions at the same time — used of a plan, campaign or attack that goes at a problem from two sides at once.
雙管齊下的
adjective · B2
Sharp on both long sides, so the blade cuts whichever way you move it.
兩面開刃的