Would Jon really have killed Dany? First, we have to define a frame of reference for Jon. Since I haven't entirely read the books, and since there's a clear delineation in the series where Martin's direct inspiration drops off and the writing style changes, I'll define the real Jon as Jon in the first 6 or 7 seasons of the series: "6-7 Jon".
Jon is motivated by family, love, and transcending his bastard identity, but perhaps above all, he's motivated by the honor that drove his father. He's not a Varys, who always purely "served the realm". Therefore when Tyrion asks him to do "a terrible thing, but the right thing", Tyrion is speaking in the best interest of the realm, and therein lies the rub: is the right thing the best thing for the realm, or the honorable thing for the individual? And therein lies the rub: what's the meaning of this code of honor, if it doesn't serve the greater world? Why would Ned refuse Joffrey's claim, if seemingly everyone suffers in his refusal? Is that honorable, or selfish, or arbitrarily religious? Why would Jon keep his queen when seemingly everyone suffers for it, nor does she have the best claim? And therein lies the rub: does he break his oath for the good of the realm, or because her claim was proven false; and why do he and Ned care so much about traditional lines of succession; and if they do care so much, why does Jon support the Targaryen succession over others before or after them; and why does Jon continue to speak, to the very end, like Dany is his queen, when he plans to topple her and when lying is a cardinal sin in his mind? I think the most interesting question is the meaning of this "honor". I suppose it means keeping oaths, but why is that the summum bonum? I think it's a good code, in that it's simple and it generally serves everyone. If you're going to live by a code, you could do worse than this. And I get the idea of living by a code: you don't have to second-guess every decision, and your conflicting drives don't corrupt your actions. But true wisdom is not codified like that; it's fluid from moment to moment; it's spontaneous; you have to study each new moment with supple reasoning. The wisest people don't live by such codes; this is what separates Tyrion from Ned/Jon, and even Littlefinger from them. Tyrion is constantly considering the best outcome for the party he's serving; it's exhausting but it gets better outcomes than following a simple code, if you're smart enough and rational enough to keep it up. Littlefinger takes it even further, explicitly stating that he has one goal in mind (the Iron Throne with Sansa beside him), and he plays out every possible action in his head, assessing whether it satisfies that goal; but Tyrion is a better example because his goals are not so simple, not so self-serving. Tyrion is probably the best example of wisdom among the main characters (not pulling from Maester Aemon or anything).
So I'm saying Ned and Jon are not the best examples of wisdom. Their codes are simple and intuitive, which gain them respect and, to a degree, success and a sense of maintaining world order. But their success is characteristically neutered, as is their service to the world, and they surely don't find happiness through this code. Happiness and service cannot be cleanly codified; the world, and the soul, demand supple decision-making. If someone isn't brainy enough for that, a code such as honor isn't the worst they can do; it probably lands them in a better spot than the improvisation of Tyrion and Littlefinger would. Littlefinger himself says the Starks are slow, Ned calls himself primarily a soldier, and Jon says he doesn't like riddles. Ned and Jon are relatively straightforward, so a straightforward code serves them. I do think they're wise, but too pious to be wisest.
Anyway, 6-7 Jon wouldn't have killed Dany because his job isn't to serve the realm at all cost (unlike Varys), and killing his sworn queen is more of a breach of honor than it is an upholding of his oath to be "the shield that guards the realms of men". Alas, classic case of conflicting oaths. But I think the clearer path is to not murder your sworn queen, as "guarding the realms of men" is ambiguous in the first place.
Then again, Jon interpreted his oaths loosely when he lay with Ygritte and when he abandoned the Watch after his death. He's half Targaryen after all. But breaking his oath to Dany was too egregious, I think, for 6-7 Jon, so either Jon authentically evolves in S8 or, more likely, the writing fails to deliver authentic actions or at least to properly justify his actions. Most citizens of Westeros would egregiously break an oath for their own good or for the realm's good; but Jon isn't most citizens.
Would Dany really have decimated King's Landing? It's strange how instantaneous that decision was. As the bells ring, you can watch her expression turn to slaughter. Why, when the surrendering bells ring, do her eyes begin to smolder? That's identically the moment you relent, especially at the explicit advice of your counsels. But tell me too, why does she raze the whole city? I can see her attacking the Red Keep, that is, cutting the lion's head, but why burn the whole thing? How will that secure power for you and peace for the people in future generations? You're destroying so much infrastructure you'll have to rebuild, and decimating an innocent population who never swore for Cersei. You're creating fatal enemies for yourself (Jon, and probably others if Jon cowed away from the deed). You're mimicking your loathed father by setting fire to King's Landing. You're spinning the wheel you intend to break; how will that not make it harder to break? And why does all of that aggression fall on that one moment, the moment your enemies surrender? The power of sacking the capital just kindled her desire for more power? It feels out-of-character. Now, I saw her fury at Missandei's execution; her despair when her second dragon went down; her sense of betrayal at Jon's revelation, both the betrayal of her advisors for spreading the secret and the strange betrayal of the fates for lifting someone into the seat she was always made to occupy. She is angry, and more importantly, confused -- confused at her own destiny, in light of Jon's new identity, and she's eager to coax that destiny into existence, the one she's believed for years.
When I first saw S8, I thought these two actions I'm discussing -- Jon's regicide and Dany's massacre -- were uncharacteristic and unjustified. I still think so, but I don't think the show does too bad a job at supporting them. Jon's action is supported through his trend of interpreting his vows looser than Ned did his, and through Jon's revelation as a Targaryen, which endows him with a higher destiny. Even if he doesn't believe Targaryens are actually nobler, he knows they are more intertwined with nobility and with the fate of things than Starks are. He must have an elevated sense of worth and fate, knowing he's a Targaryen-Stark heir rather than a Stark bastard. So it isn't insane to think he would suddenly start reaching for the future prosperity of the realm as opposed to keeping to his simple humble code. As for Dany, her destiny is suddenly threatened in the person of Jon, and she sees how much they love him, so she resigns herself to the tactic of fear. Not only is she furious, she's desperate and confused, desperate to manifest the fate she always believed in. So in that way, she's a different person in S8 than prior seasons. Still, I'm skeptical either of these critical actions would have been taken by the characters as they were built up in the first 6/7 seasons. Jon is too honorable, and Dany is too kind. Her greed and desperation shouldn't have withered her heart so suddenly, and only a withered heart burns the capital to the ground with all the innocents in it.
