What about a lawfully deposed king (I am sympathetic to the criticism of the actions of Cromwell/Parliament by @
alexandermsmith)?
As I stated above, when David was anointed as the king to replace Saul, who had been rejected by God as king ("The Lord then said unto Samuel, 'How long wilt thou mourn for Saul, seeing
I have cast him away from reigning over Israel? fill thine horn with oil and come, I will send thee to Jesse the Bethlehemite: for
I have provided me a King.'" I Samuel 16.1), David was immediately empowered: "Then Samuel took the horn of oil, and anointed him in the midst of his brethren. And
the Spirit of the Lord came upon David, from that day forward" (v.13).
I don't believe this can be disregarded as simply "OT uniqueness." When Paul states in Romans 13 "Let every soul be subject unto the higher powers: for there is no power but of God: and the powers that be, are ordained of God. Whosoever therefore resisteth the power, resisteth the ordinance of God: and they that resist, shall receive to themselves condemnation," this is a universal declaration. Saul was "un-ordained," resisted the power of David who had been ordained in his stead, and therefore was condemned. So does a person in such a position really have to "do complex justifications"? I'm not sure how this position and the explanation of @
Laborer for the Lord answers the question of why David could not justifiably kill Saul once he been deposed and thus was an enemy of both God and his nation.
It is interesting that there is no comment elsewhere in Scripture - despite similar comments like "righteous Lot" - about David not killing Saul, but we do have Paul reiterating that Saul was removed from his office and David made king in Acts 13: "...they desired a King, and God gave unto them Saul, the son of Cis, a man of the tribe of Benjamin, by the space of forty years. And after he had removed him, he raised up David to be their King...."