something that has both positive and negative aspects
—Becoming an adult is a double-edged sword: It brings a lot of freedom but a lot of responsibilities, as well.

Most of Solomon’s proverbs are collected in the Book of Proverbs, which contains the following caution:

My child, be attentive to my wisdom,
pay close attention to my understanding,
in order to safeguard discretion,
and that your lips may guard knowledge.
For the lips of the adulterous woman drip honey,
and her seductive words are smoother than olive oil,
but in the end she is bitter as wormwood,
sharp as a two-edged sword. (Proverbs 5:3,4)

Here the important quality of the sword is that its two edges are extremely sharp. But in modern usage, two-edged has become double-edged, stressing the sword’s ability to cut in two directions.