But even if that nonsense about the phalanx was right, what Leonidas actually did about the secret path was even worse: absolutely nothing. No wall of boulders to block it off, no traps to decimate the Persian troops, not even guards to send warning back to the troops. Look, even if the hunchback hadn't made like Lando Calrissian, Persian scouts might have found the passageway all by themselves. Instead of making literally any effort to stop them, Leonidas pretty much just forgets about the whole thing. We never even see him ask about the path's location--for all we know, he has no idea where the damn thing actually is.
There's a line between heroically facing death and actively courting it; Leonidas sprinted over that line and never looked back.
Specifically, he first sprinted in fast motion, then slow motion, then fast motion again.