O.K. I get the scenario. If a POTUSA were to openly or by preponderance of evidence known to shoot, knife or by whatever means commit murder, POTUSA would have to be impeached and removed from office before being tried before a jury of his/her peers for the crime of murder. This seems to be a convoluted procedure for justice and not how the ‘common’ person would be treated. I don’t believe a POTUSA with the public and political support of a person as for Obama would ever be impeached and removed from office for whatever crime and as such is shielded from equal justice. The King is almighty.
Murder (not killing - as in self defense) is not anything that can be done in pursuance of the duties of office. Acts done in the pursuance of official duties may have immunity, these are official acts. There is no immunity for other acts.
Whether impeached or not would be moot, he would be prosecuted for murder.