"But I have to tell X what they're doing wrong. Or I'm not being honest!"
Candour is one aspect of honesty. But it's not the only one. When I don't disclose something, I'm not being candid, but that does not mean that I'm doing anything wrong.
Candour is not an unalloyed virtue that trumps all others. In society and in relationships, we do not say everything we think to everyone at all times. If we do, we are rightly considered disordered, dysfunctional, and/or dangerous.
Why do we not disclose everything to everyone? Because the purpose of communication is not to vomit out every passing emotion or thought at all costs: it's to serve a particular valid purpose in the relationship in question.
Honesty, in the sense of candour, must be tempered by tact, discretion, diplomacy, and expeditiousness.
Does it need to be said?
Does it need to be said by me?
Does it need to be said now?
It is true, necessary, and kind?