In my ethics class, we were taught to look for an "imminent" threat to self or others. I have a tendency towards thoughts of suicide but I have never attempted it nor have a plan. So, I would have a lower risk for suicide. If the person is of a higher risk, one could do a non-suicide pact and increase session. Pershaps the T could call the client's spouse in front of the client. If the person has a plan, past attempts, the means to do it etc, it might be advisable to recommend hospitalization and move to involuntary if the threat is "imminent." I think some states that have the word "imminent" before threat when it comes to suicide and homicide.
I don't think we discussed a previous crime like murder. I think I would first try to see if the client might try to kill again. I would be required by law to protect people if he/she was an "imminent" threat. If not, I would probably encourage the person to turn themselves in because it might help them to relieve the guilt. However, I would propably not discuss it with anyone except my clinical supervisor/client's Pdoc if in same agency. If the client is not an "imminent" threat, I don't think I could justify reporting it.