I wouldn’t be surprised if in Book 2, Leona had dehumanized Malleus so much that he didn’t even really think Malleus would die from the stampede. He describes Malleus as a monster earlier in the book, so in his head he could have been thinking “He’s fae, a few people won’t kill him… it’ll be just enough to stop him from participating in the tournament.”
Because Leona’s smart. It’s easy to forget that with how Book 2’s writing is, but Leona is one of the most intelligent members of the cast and is keenly aware of how politics work. That’s kinda the point of his dream in Book 7. So he of all people should have known what would happen should the one and only heir to Briar Valley die. It would be a political incident like no other. A Leona fully in the right headspace would never try to cause an incident on the level of murdering a future monarch.
But to him, Malleus was not a person that could die. He was a monster. He was a symbol. A symbol of all the things Leona himself could not have. When Leona looks at Malleus, he doesn’t see Malleus. He sees a crown, forever out of his reach. That crown drives him mad.
It’s that madness that drives Leona in Book 2. Not loyalty, not justice, and definitely not the intelligence he prides himself in. Just pure, mad, rage.
Like he says before he overblots: “How could any of you understand my anger? My pain?”
I think part of Book 2’s problem is it severely understates just how much anger is steering every choice Leona makes. He’s not being smart or cunning. He’s just an angry kid, throwing toys until someone gives him what he wants. But they don’t point out at all how the oversights in Leona’s plan are out of character for him, it simply passes it off as his usual cunning, so he looks stupid in the process. But he’s not stupid, his rage is simply overriding his brain.