#printfogey

They surely both did say “We're gonna do everything it takes to bring you down!” (or similar, it depends on the subs clearly).

Would have they? Or better, have they? We canonically know the answer to this question(s).

lt; nr –– Extremely brief synopsis:

Neither Gintoki nor Katsura meant to kill Takasugi. Katsura wouldn’t, Gintoki would have if necessary, but rather not.

I don't think it is needed to spend too many words on the fact that Gintoki actually (almost) kept his end of the promise. Though obviously, he was not willing to face Takasugi, which meant to face the past (but not healed) trauma from which he has never really recovered. We know Gintoki is more prone to avoid to recall it until he has to (while Zura and above all Takasugi tend to dwell on it, though in different ways).

Gintoki is also the one who, as Katsura and he were menacing to wage war on Takasugi, strongly discouraged the latter to come across one of them. And here he wasn't lying, though he wanted to pass it off as a threat, but honestly I think the crucial reason behind it all was selves-serving (I used the plural because he spoke, knowingly or not, both for him and Katsura): that being the case, better not to have anything to do with you because we ought/might have to do something we don't want to do. And, honestly, the joint declaration too aimed to this.

So... I'll focus my speech mainly on Zura, being he the more controversial and being I more comfortable with his character. I hope all this holds water!

Katsura as Shouyou's “surrogate”

Katsura is basically, in a way, a “double” of Shouyou. This is self-evident if we merely look at them from a sheerly outwardly standpoint. But, no matter how much their features could be similar in the past, after Shouyou's death, Katsura took Sensei's shape. Loosen hair, a formal, plain kimono and haori. He honours him, he mourns him. It is a result of the genuine admiration Zura had and has for Shouyou. We know Zura tends to wear masks - he is so Pirandellian!).

He “overlaps” their figures.

This is obviously not just a “fashion choice” (though it's the kind of clothing that quite much fits with his more traditional samurai-sh part). I heavily believe that it is above all an attempt at fulfilling the role Shouyou had left uncovered. Maybe, at the very beginning, also for himself, as a coping mechanism.

I really like to joke about his “motherly”/teacherly attitude, it is funny, but I also think this is partly because of the said role. He surely has always had this attitude even as a child. In this, his pre-Shouyou life and grandmother's influence/education (“Be a general”) may have played a prominent role, but probably that laid the foundation of the whole matter to be even more heartfelt in his adult age.

Consider what Katsura says in Chapter 689 when he is talking with Takasugi. He tells him that he wants to protect what sensei left behind. Which is also, and above all, his students. We could argue that to Katsura sensei's legacy were Gintoki and Takasugi. And Zura asks him to stand aside (and I think this tells us something).

That's why I'd take a step further. Not only Katsura has always agreed that Gintoki took the right decision, but that is also the decision Katsura would have wished if he himself was Shouyou and the why of his silent self-sacrificing part. Though it may seem going astray, I'd add also the moment in Ch. 699 when Zura shoves Takasugi away along with Gintoki being him to be struck. In my opinion, this strengthens the thesis I'm bringing forth.

Note that here Zura is wondering about sensei's thoughts, get inside his mind.

Katsura's absence in Shogun Assassination arc

The whole fight between Gintoki and Takasugi was clearly an issue they had to resolve on their own. Katsura was lying near Takasugi when Gintoki beheaded Shouyou to save them. I don't think I've ever seen a line in which Shinsuke said anything to Kotarou about the fact he had to do something. He just tells Gintoki, during their fight, “they [Katsura and he] weren't meant to survive and Gin should have saved sensei and abandon them”. Although  Katsura wouldn't agree on this.

But I can't help but think that one (even if not the only) reason Zura wasn't there is that he wouldn't have been able to stand and watch his "putative brothers" fighting and almost killing each other. He would have physically interfered between them, maybe. Despite everything and that he saw Takasugi as an enemy by then.

Sensei's sacrifice not to be wasted

Just dropping here the consideration that he isn't ever strictly involved with Oboro (even when they were near – Farewell Shinsengumi and Rakuyou arcs – they didn't meet). One of the most recurring Oboro's sentences to his “kohai” was to not waste sensei's sacrifice. Maybe this line wouldn't apply to Katsura?

“You just wanted to save your old comrade Takasugi”

We are back at Benizakura arc (finally!).

He went there with the intention to save Takasugi, as Katsura's men pointedly said. And he went there alone saying it was his problem, not even informing Gintoki. I argue because he can't (nor probably wants) shake off the previously said legacy.

It's something different from what Gintoki said to Shinsuke during their fight about to protect Takasugi's soul as a Shouka Sonjuku disciple.

My guess is that Katsura means to preserve both life and soul. An evil comrade is better than a dead comrade. Out of sight and possibly out of mind (but not succeeding, he refers to him much more than Gintoki and not in aggressive tones – I can think at least of four moments), but that part of heritage still lives on.

But wait! Katsura actually cuts him in Benizakura!

I  find that is the equivalent of a mother beating his son on the hands with a ladle. Jokes aside, Shinsuke was clearly blindsided, completely caught out of guard. Petrified, I daresay. Because he thought Katsura was dead by then... But maybe also because he wouldn't expect such a violent deed on him (or Gintoki) by him? He would have the time to react (this is clear in the anime, not in the movie) after recognizing Katsura's smell and voice, but it seems he can't. I think this is the only time we see him not having a ready reaction.

It wasn't a fatal injury anyway, nor it was meant to be it I'd say: after that, even when they were alone, he doesn't try to have a physical confrontation. Even right at the moment when he discovers he has a deal with the hated Harusame.

We should consider, that both when he “cuts” Shinsuke and when he swears to kill him, anger could actually have had an influence, but this doesn’t mean they were whole-heartedly felt words.

That's why I don't take seriously Katsura's threat to kill Takasugi. But there are, and we have to consider the fact that he told so anyway at least twice (plus in Benizakura).

Zura nobody believes you.

This is even funny because they weren’t even really enemies by now. But habits are hard to kill...

Some clarification(s):

It’s not that Zura wouldn’t act, even violently, against him if Shinsuke really went too far even for how he is. But he’d use more a verbal approach and, on a physical end, “disable” him rather than act in a way could really kill him. Differently, whichever was Gintoki’s belief, his battle with Shinsuke was so rough and violent, brutal, that well they were both lucky to survive to that.