Was John being smart here?
My first impression of John's decision to kill Santino here was that he was very emotional, and that impression has stuck ever since. And I don't deny that a big part of it was emotional, but when I go back to it and think through his options, was he actually aiming for the best one?
In my opinion, beyond John's "focus, commitment and sheer will", another characteristic that he has that's often overlooked is that he's incredibly time-efficient (though that doesn't roll of the tongue too well).
Going back to the scene where John kills Santino; let's imagine a scenario where John didn't pull the trigger. Santino, now that he has taken a seat at the High Table + his already large influence and power, he will definitely open another contract on John. Worldwide, even. The bounty will remain open indefinitely until the job is done. After all, even though Santino boasts about how "you wouldn't have the same meal twice" in the Continental (and the hotel will provide him safe refuge on paper), obviously he doesn't want to stay there forever. He would want John dead ASAP so he can do other things. Plus, he would likely still fear being in the same building as John Wick.
John, on the other hand, while he is still technically safe inside the Continental, we already know what happened in the first movie. There are literally people who are willing to break the rules and be denounced excommunicado (i.e. Perkins). Knowing this, John isn't safe in the Continental either and will always have to watch his back. Even if we exclude those daring outliers, who knows what bullshit Santino could conjur up just to get John killed, moreso now that he has a seat at the High Table. While Winston could help somehow, he is still beneath the table, so I'd say the help he could give would be very minimal.
So, in John's mind, if everything will go to shit anyway, and if he will be hunted down anyway, he might as well have it be now.
This "time-efficiency" is also very consistent of what we've seen so far of John. In the third movie, after he went back to New York from visiting The Elder, we see him charging towards the exit door and tackling the motorcyclists just after hearing the motorcycle's roars (in his mind, he probably thinks it's better to attack now rather than staying on the defensive). In the fourth movie, the time he asserted for the duel is literally "Now." After all, Marquis said it himself, "how you do anything is how you do everything." This behavior reflects a lot in his actions inside and outside of combat. There are other examples that I have a hard time explaining without a video
The only time I think he strays away from this characteristic is when he's hunting Iosef. He definitely wanted to take his time with that one.
To be honest I'm quite unconfident on the term "time-efficiency" that I use here, but I'm basically referring to his characteristic of "might as well do it now even if it sucks" or something like that. Which I find very cool and admirable, just to bite the bullet like that with full confidence. Let me know your thoughts