Itachi is not wrong that the idea that this man wanted to leave him alive to send a message that did not rely on Itachi, specifically, bearing it, technically does assume less that must be true.

But - and it takes her a minute to figure out why her instincts broke this way, but when it does much becomes clear to her - this man's actions are pinging her Sith Lord Revenge Plot Detector.

And Sith Lord Revenge Plots™ are never, ever, that simple.

<...You are right that those other possible explanations are more parsimonious, and that I likely should have considered them first, starting from a state in which I know that I know nothing.>

<And yet.>

<And yet.>

<...Too much is just too strange about the circumstances and results of this, Itachi, for me to consider anything about this event as likely to be simple.>

<If this were my world, I would suspect the Force of meddling.  Of there being some critically important locus within this event on which the galaxy turns.>

<This is not my home.  I have no reason to affirmatively believe my presence here is anything but some absurd cosmic coincidence.  I have no reason to believe that I should listen to the instincts that tell me that this plot is kin to the sort of deliberately maximized cruelty that one of my maddened and power-drunk 'colleagues' might execute when they believe themself slighted, no matter the disquieting similarities in fashion sense and the shared dramatic flair.  But I do not think we should disregard the hypothesis that he wanted something from you, merely because there are other, nominally simpler, explanations.  And I think that the simpler assumptions of this man's state of knowledge add just as much assumptive burden, if not more, to his reasoning for acting as he did.>

<What message could this possibly send?  Neener neener I can kill you?>

<How does he benefit from leaving you alive, in the universe where he didn't have investment in anyone in particular surviving?  It trades his anonymity for...  What?  What did he gain from the death of the daimyo not being a horrible accident?  It could be some aversion to hurting children, but your teammates were not much older than you, were they?  And I suppose he could be trying to start a war, for...  I haven't the slightest clue to the geopolitics, and will have to leave that to you - but then why did he seemingly show no obvious allegiance - not even an obviously, or less obviously, false one?  You just don't do false flags under no flag.  So what could he have wanted someone to see, that hasn't been undercut by his not taking the opportunity of having you at his questionable mercy, to tell you to observe and report - not even as a bluff?>

<...And, to address the other prong of your simpler hypotheses - I don't think he thinks he owes your clan, not if he is going to leave you with the blame for surviving where all else died.  That leaves us with some three-consequences-deep scheme to affect the circumstances you're working under as another possible motive, and that of equal implausibility and fickler impact, but...>

<He seemed focused on you.>

<He made sure you saw.>

<Why would he do that, if it wasn't important to him?>

<If nothing else, where in the stars' name could he have come from?  Assassins do not sprout on trees, and their training should leave traces.  And why pick now to show his face?  Or rather, his oddly decorative, and clearly properly custom-made, mask.  I'd say we should find out who made it, but they're likely either firmly unavailable for us to interrogate, or dead.>

<...Actually, speaking of that mask.>

<Do you know of any relevant symbolism it could be referencing?  Because he put effort into having it.  It likely means something.>