Okay so I don't normally do this, but I find this topic fascinating so I wanted to blab a bit. See, some wonky execution aside (mostly I think from Books 1 and 2 being committed to being too fast-paced), I think it was obvious that Leona was only trying to appease his dormmates back in Book 2, because he says as much then. Keep in mind, what specifically started to trigger his overblot was *not* the plan failing, but rather, when his dormmates cried out to him and called him their leader. That's when he starts to get irrationally mean, and mocks them for ever believing the plan could work. Then it was a two-hit combo with Lilia saying he was not a fit leader like Malleus, and that's when Leona waxes poetic about never being king.
This is not, however, his true snapping point. Keep all this in mind.
Leona fascinates me because he's a very interesting line between a filthy liar, even to himself, and being incredible self-aware. And I think part of this is due to the fact that two somewhat paradoxical things are driving both his rage and depression (because OP is absolutely right that rage is in the equation, albeit underneath the nihility).
These two things are both key factors: Leona can never be "king", and Leona is terrified of being a leader.
He leans into the first, which is in actuality misrepresented, but in Book 2 is overblot is actually largely fueled by the second. SavanaClaw believed Leona could be their savior, but he did not believe this. I do think Leona genuinely tried to help, but then once again, there's this insurmountable obstacle, a born prince actually destined to be a king, who's very existence is stopping every plan, and Leona just. gives up. But because he gives up, he is then failing the very position he once believed he would be good at, and when this is brought to attention, he just can't take it anymore. The paradox collides, and he overblots.
A large part of Leona's damage is in his hometown event, not Book 2. Sadly I think Book 2 was way too committed to mirroring The Lion King, and later TWST gets to breathe and exit this formula a bit more. However the hometown event provides loads of context, as well as the crux of his real issue: nobody at home has and will ever listen to him, because they are resistant to change, even if their ways are actively hurting the community. It's not about "being king." It's about not being able to make a difference, about being ostracized for being genuine, but this is a vulnerability that Leona cannot and will not articulate. So we have to read between the lines.
In Book 7, his "dream" is also his nightmare, because that paradox is the driving force of his motivations. He is finally able to influence his country, and he does terribly, because his greatest fear is that Neji and everyone is right, and his intelligence is a sham and his personality is a poison. This is the same fear that set him off in Book 2. He's finally in a position of power and he can't do a damn thing. He's still useless, but instead of being ignored, he outright failed, and he could not take it.
We got a crucial look into his mentality in Book 6, in his conversation with Jamil. He insisted that Jamil gave up too early, and that you have to "put your blood, sweat, and tears into it, and then you give up." Because that's what Leona did. He tried a lot, based on what seemed to be a very well worn argument between him and Neji, and then he failed. And he gave up, and now he is beaten down and afraid of trying again, but it's easier to act apathetic than to admit—to others or himself—that he cannot bear to actually fail if he gave it his all. So he stays stuck in a middle ground forever.
Book 2 is the perfect storm. His dorm is largely reliant on spelldrive for post-school opportunities: it is a problem, and they believe Leona can fix it. This is a meaningful situation. (And, his emotional damage aside, his method to this is absolutely scheming and foul play here. He's placing his dorm above everyone else and justifying playing dirty because of the stakes.) However, because Malleus is that formidable of an obstacle, the nihilism sets in fast, and he doesn't help that Malleus is a picture of a lot of the things already triggering to him: he's a prince, he's well loved by his people, he's powerful, and he has everyone's ear. (This is Leona's perception, of course.) He's clearly superimposing some stuff on Malleus once the lack of rationality kicks in and that's part of the problem. Malleus is both a picture of the insurmountable system that has quashed him and a royal like his brother (who, let's face it, is not like Mufasa and is, in fact, not a great king. see aforementioned infrastructure issues).
Now, the stampede will not have killed Malleus. Full stop. He can fly, he can teleport, he can punch through stone with his bare hands, and knows loads of shield magic on top of being physically built like a tank. I think Leona was fully aware of this, because this was where the plan fell apart and he gave up. It still bothered him very deeply, however, and that is where the real rage starts to shimmer in the background, with Malleus as the target, not the cause. There's a lot to say about the parallels and foils between Malleus and Leona, too, but that is a different blab session lol.
Leona is one of my favorite overblots to study because so much went into it, and even in the thick of it, he held back the core of the matter, because that is something he has always avoided. Leona is beyond pissed that his home country is actively perpetuating a system that does not work and also harms its own people. (i.e. his vendetta against the fountain, or the other cities not having paved roads.) He's mad that nobody listens to him and because he was born second, nobody ever will. He's devastated that he could not make a difference. He's nihilistic and believes that all of effort will be wasted and end in failure. He's terrified of failing. He wants to be listened to but he's uncomfortable when people rely on him, because of all aforementioned reasons. He wants to try; he wants to work hard and achieve something meaningful; he wants to be appreciate on his merit; he wants his existence to actually mean something. However, he stopped believing that long ago, and then it was easier to lean into being the bad guy than to come to face his heartache. (Which is the pride talking, in a way, but that's also another subject.) So having all of this thrown into his face all at once? Punctuated by Lilia repeating nearly verbatim the sentiment we see prevalent in the Sunset Savana, that his attitude (his response to this pessimism) is proof that he would have always been a bad leader? Well.
