A double-edged sword was sharpened on both edges of the blade, so that both were lethal. It doesn't mean that the sword had two blades, with one of them facing the user, but that neither of its edges was blunt.
The modern meaning of the idiom "double-edged sword" as lethal to the one wielding it is not applicable in any of the places where the phrase appears in the Old or New Testament (Judges 3:16, Psalms 149:6, Revelation 1:16), so I don't think the comparison of the word of God to a double-edged sword was meant to evoke the idea of good and bad consequences.
Looking at the metaphor in context, the verse seems to give a series of two characterizations of the word of God:
Indeed, the word of God is (1) living and (2) active, sharper than any two-edged sword, piercing until it divides (1) soul from spirit, (2) joints from marrow; it is able to judge the (1) thoughts and (2) intentions of the heart. (NRSV)
Apart from the obvious metaphor of sharpness, I think that what is emphasized by the comparison to a two-edged sword is duality: the word of God is two things, it divides two things, it judges two things.