Word or idiom for something that can either be helpful or harmful, depending on how it's used [duplicate]

Solution 1:

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)

Solution 2:

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:

• "Russian Energy: A Blessing and a Curse" (title of academic article)

• "No One's Like Giannis, But That's A Blessing and A Curse" (title of sports article)

• "ADHD can be a blessing and a curse" (title of article on school website)

Solution 3:

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.