The American Bar Association's model rules of professional conduct, which guide State bar associations and ethics committees lists six situations in which a lawyer may disclose confidential information without the implied or express consent of the client. Rule 1.6(b) enumerates them:
(1) to prevent reasonably certain death or substantial bodily harm;
(2) to prevent the client from committing a crime or fraud that is reasonably certain to result in substantial injury to the financial interests or property of another and in furtherance of which the client has used or is using the lawyer's services;
(3) to prevent, mitigate or rectify substantial injury to the financial interests or property of another that is reasonably certain to result or has resulted from the client's commission of a crime or fraud in furtherance of which the client has used the lawyer's services;
(4) to secure legal advice about the lawyer's compliance with these Rules;
(5) to establish a claim or defense on behalf of the lawyer in a controversy between the lawyer and the client, to establish a defense to a criminal charge or civil claim against the lawyer based upon conduct in which the client was involved, or to respond to allegations in any proceeding concerning the lawyer's representation of the client; or
(6) to comply with other law or court order.
The seriousness of a crime committed in the past is immaterial to the analysis. If you consult with your attorney regarding a past crime, so long as the victim is not still in peril, or you are not planning on committing future crimes, that attorney must keep your confidence.
The rules of professional conduct would prohibit a lawyer from disclosing any prior (past) criminal activity that you consulted with them professionally about, regardless of the seriousness of the crime.
If you told your lawyer that you had killed somebody, that communication is privileged. If you told your lawyer that you were going to kill someone, your lawyer would have to disclose that information in order to save a life.
My advice should in no way be construed as creating an attorney-client relationship.