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For example, consider the following:

Knives are a [blank], because they can either be used as a tool or a weapon.

I don't think "double-edged sword" is appropriate in this context because that idiom implies the object is simultaneously helpful and harmful. I'm instead, searching for a word or phrase to describe something that is either helpful or harmful depending on how it is used.

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work both ways

If something such as a particular situation or type of behaviour works both ways, it has equal advantages and disadvantages for everyone it involves

  • We are expected to be very flexible, but that should work both ways. (macmillan)
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Knives are a blessing and a curse, because they can either be used as a tool or a weapon.

The expression comes from the Geneva translation of the Bible (1561), Deuteronomy 11:26-28:

26 ¶ Beholde, I set before you this day a bles∣sing and a curse:

27 * The blessing, if ye obey the commande∣ments* of the Lord your God which I com¦mande you this day:

28 And the curse, if ye wil not obey the com¦mandements of the Lord your God, but turne out of the way, which I cōmande you this day, to go after other gods, which ye haue not knowen.

The phrasing made it into the King James Version virtually unchanged, and the expression then took on a life of it own (see the Wiktionary entry). While its uses are sometimes religious, it's not hard to find secular uses that have the gist you mean, namely that the knife could be helpful or harmful:

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Probably your best option is "can cut both ways" implying that the negative or positive effects are not guaranteed. Unlike the simple "cuts both ways" which means that the effects are both assumed.

E.g. Empathy Can Cut Both Ways

A Ritual That Can Cut Both Ways

Feeling emotionally connected to your client can cut both ways

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Mixed Blessing

something that has advantages and disadvantages

Getting into the team is a mixed blessing - I'll have to spend a lot of time training.

[Cambridge Dictionary]

You could therefore say Knives are a mixed blessing, because they can either be used as a tool or a weapon.

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“A good servant, but a bad master”

This proverb originally (1615) was used in a comparison with fire, and I learned it in this respect as a schoolboy (many, many years ago). It can apply equally — and, in my opinion, quite elegantly — in this case.

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