I've read the Lieber Code - 'unarmed resistance by citizens of the United States against the lawful movement of the troops is levying war'. Of course the military determines what is lawful, eh? Is failure to provide food or shelter 'levying war'? Lieber invented this tripe to EXCUSE the atrocities committed by Union troops.
Regarding Dahlgren, you wrote:
His "orders," as can best be derived from the prepared speech to the raiders, was to free Union prisoners, to burn Richmond, and disrupt the confederate war effort - which is a reasonable and lawful military goal. From the "speech": "We hope to release the prisoners from Belle Island first, and having seen them fairly started, we will cross the James River into Richmond, destroying the bridges after us and exhorting the released prisoners to destroy and burn the hateful city; and do not allow the rebel leader Davis and his traitorous crew to escape.""[T]o burn Richmond ... to destroy and burn the hateful city ... do not allow the rebel leader Davis and his traitorous crew to escape."
Clueless, what part of that do you not understand?
That part of Dahlgren's speech provides no suggestion of assassination. Indeed, it suggests that the first option would be to capture them. Of course, if they were shot trying to esacape ...
As far destroying and burning the "hateful city" of Richmond, that seems to be a legitimate military goal. Not only was it the site of the insurrectionist government, but it contained numerous factories, warehouses, and armories that aided the war effort. In many ways, it is too bad the confederates burned it to the ground before the Union had the opportunity.
"Of course the military determines what is lawful, eh?"
Where the Laws of War, rather than the rule of law, prevails - YES.