Certainly the root cause might be fear and insecurity, but he still acts it out externally as if he is underestimating his opponent, so the physical results are the same. The cause might differ, but it ends similarly.
Anyways, I think Itou shows us what further along the path of 'unrivaled under heaven' or the path of carnage looks like. The dark-side (or what every you want to call it) is the metaphorical symbolization of that path. It doesn't adapt laterally with the outside world but rather confronts and over powers it. That is to say its attributes are static, but it's intensity is variable. In following, Itou seems to not have grown much philosophically but does seem to have grown in his intensity and ability to wield it effectively. I think Itou might be meant to show the folly of acting youthfully in ones old age. Looking back, young Musashi and Itou act similarly and the urges that motivated those action now seem manifest in their dark-sides. As we can see by the size and maliciousness, Itou's intensity is massive, but perhaps there is an upper limit to it or it can never attain a large enough size to overcome the world more broadly. Musashi, on the other hand, is at least thinking about moving more laterally in his life path. Itou's intensity wins this time, but not by a wide margin, thus his current predicament. As far as Musashi can tell though, Itou's path has paid off, thus he is temped to follow him. I think Itou is meant to show that that path is tragic on at least some level. Besides the underlying insecurity you mentioned, at some point he lost part of his hand and to a swordsman, that is a tragedy.
it shows that intensity is fickle and fleeing, thus reinforcing the point that Itou's [and potentially Musashi's] path is fundamentally flawed.
Anyways, I think Itou shows us what further along the path of 'unrivaled under heaven' or the path of carnage looks like. The dark-side (or what every you want to call it) is the metaphorical symbolization of that path. It doesn't adapt laterally with the outside world but rather confronts and over powers it. That is to say its attributes are static, but it's intensity is variable. In following, Itou seems to not have grown much philosophically but does seem to have grown in his intensity and ability to wield it effectively. I think Itou might be meant to show the folly of acting youthfully in ones old age. Looking back, young Musashi and Itou act similarly and the urges that motivated those action now seem manifest in their dark-sides. As we can see by the size and maliciousness, Itou's intensity is massive, but perhaps there is an upper limit to it or it can never attain a large enough size to overcome the world more broadly. Musashi, on the other hand, is at least thinking about moving more laterally in his life path. Itou's intensity wins this time, but not by a wide margin, thus his current predicament. As far as Musashi can tell though, Itou's path has paid off, thus he is temped to follow him. I think Itou is meant to show that that path is tragic on at least some level. Besides the underlying insecurity you mentioned, at some point he lost part of his hand and to a swordsman, that is a tragedy.
If Kojirou is able to be the one that takes his hand, even if he loses afterward,