Disclaimer: this is not legal advice. Plus I'm not going to claim expertise on ACP, particularly in a private practice setting in Canada, so this is JMO on general background. There are a couple of layers here, and they will vary among jurisdictions:
1. Rules of Professional Conduct: As a general rule, attorneys have a duty to protect the confidentiality of client communications, but each jurisdiction is going to have its own exceptions. In my state, the exceptions are generally aimed at preventing future crimes or harms (not about solving past crimes), and there are no broad exceptions for the client's best interests or substituted judgment about what the client would have wanted. Jurisdictions may differ about whether ACP survives the client's death, and what is a communication in relation to representation. For example, in Windsor's hypothetical above there could be an argument that John Smith's communications about the threats made by Bill Jones were incidental and weren't communicated as part of the attorney-client representation if Smith were consulting the lawyer for unrelated reasons.
2. Admissibility/Rules of evidence: Each jurisdiction has different statutes and/or court rules on admissibility of attorney-client communications, and the language of the statutes/rules and the case law interpreting them will differ. Hypothetically, you could have a scenario where the RPCs permit the attorney to disclose information but it might not be admissible in court, or vice versa.
If I were a wills/estates attorney and had information about a proposed change to a will that would be relevant to the investigation of my client's murder, I would seek an advisory opinion from my bar association on whether/what I could disclose.