I agree that Saul and Charles I are in different categories due to being in different dispensations of the covenant of grace. However, I am cautious about justifying executing someone for tyranny - Charles I's death warrant simply stated he was guilty of "high Treason and other high Crymes."
Tyranny has a pretty broad meaning. If the "high Treason and other high Crymes" Charles I was accused of included capital offenses, I have no quarrel with a death sentence. But someone can be considered a tyrant without blood on their hands, in which case I don't think there is a justification for executing them - remove them from power, yes, but a death penalty, no.
By the way, I do think Charles I had blood on his hands, but I am pretty sure Parliament and Cromwell were primarily motivated to execute Charles I to prevent any more heirs from being produced.
A bigger question that has long been in my mind was whether or not David was right in not executing Saul. Who else should have done it? David was anointed to take his place, Saul had blood on his hands, and the people of Israel and David suffered for it. For example, after David's restoration, "there was a famine in the days of David, three years together: and David asked counsel of the Lord, and the Lord answered, 'It is for Saul, and for his bloody house, because he slew the Gibeonites.'" (II Samuel 21.1 - Scripture doesn't record this slaying, but it is likely referring to Saul's killing of the priests at Nob in Gibeah for helping David in I Samuel 22, which some like
Gill state understood from Jewish tradition). David ends up handing over 7 of Saul's kin to the Gibeonites to "hang them up unto the Lord in Gibeah of Saul" (II Samuel 21.6) to seemingly stay the famine. Couldn't all of this suffering have been avoided if David had killed Saul in I Samuel 24 or 26? Romans 13 is not absolute as regicide is not forbidden - Jehu slew two kings at God’s appointment in II Kings 9.24.
Is there anywhere is Scripture that states David was right not to kill Saul? When Abishai wants to kill Saul, telling David, "God hath closed thine enemy into thine hand this day" (I Samuel 26.8), David replies, "Destroy him not: for who can lay his hand on the Lord’s anointed, and be guiltless?" (v.9) But then David turns around and says, "As the Lord liveth, either the Lord shall smite him, or his day shall come to die, or he shall descend into battle, and perish." (v.10). So:
- Weren't David and Saul at war (see 1 Samuel 23.8) which would justify David killing him?
- Isn't the answer to David's question above ("Who can lay his hand on the Lord’s anointed, and be guiltless?") in I Samuel 26.9 simply this: the one who God anoints?
- How would killing Saul covertly in a cave be any different than Jael killing Sisera with a tent peg after telling him "Turn in, my lord, turn into me: fear not" in Judges 4? Jael is praised in Scripture for killing Sisera: "Jael the wife of Heber the Kenite shall be blessed above other women: blessed shall she be above women dwelling in tents." (Judges 5.24). Where is David praised for not killing Saul?
Wouldn't David have been right to kill Saul by pursuing "lawful endeavors, to preserve the life of ourselves and others" and "comforting and succoring the distressed, and protecting and defending the innocent" (WLC 135)? Wouldn't David killing Saul be a "case of public justice, lawful war, or necessary defense" (WLC 136)?
Again, this was not a personal vendetta - Saul had already been "cast away" by God in I Samuel 15: "Because thou hast cast away the word of the Lord, therefore he hath cast away thee from being king." (v.23) "...for thou hast cast away the word of the Lord, and the Lord hath cast away thee, that thou shalt not be King over Israel." (v.26) ..."The Lord hath rent the kingdom of Israel from thee this day, and hath given it to thy neighbor, that is better than thou. (v.28). David was anointed to replace Saul 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), and when he was anointed, 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).