"today is a good day to die!" - Showing a lack of understanding of the responsibility he has to his people. If his dad dies tomorrow, he's king. And he doesn't know the value of his own life, or the instability his death could cause. His country supports this. Not necessarily evil if no one's ever pointed out how stupid it is to have your prince in a place where he could easily get abducted and then impersonated by a magician from a rival power. Except that sort of happened. Meaning that, assuming J4's VO isn't just outdated lorewise, he values his personal pride over the wellbeing of his nation.
Yes they have pride and a strict "don't retreat/don't fuck up" policy but i view that as them simply pushing everyone to be their best, like a father motivating a son."
and
"life is sometimes worth dying for." - Are you referencing old Taric lore? I'm not familiar with it. But I do know that his new lore paints a different picture of Demacia:
The young warrior was drawn instead to the fragile wonders that give life meaning, not endless brutish contests for flag or crown.
This was a potentially treasonous philosophy, especially for one of Taric's standing and role within the Demacian military hierarchy
You want to celebrate life rather than conquer your neighbors for glory? Well, we're going to give you "Demotion after demotion"
After his squad dies:
Taric was stripped of his rank and sentenced by Garen to endure 'the Crown of Stone', a ceremony that demanded a dishonored soldier ascend Mount Targon, known to all as a death sentence, as few mortals had ever survived the climb.
So Demacian military didn't train Taric or his soldiers that they should have guards at night keeping watch. They punish Taric for the result, and the punishment is public shaming, torture, and presumably death. That's not reformative. That sounds like the sort of punishment that might be deserved by treasonous minister that causes a few thousand deaths. Not the punishment you give to someone who failed on an intentionally trivial duty because of the ineptitude of the soldiers he was sworn to lead.
Going back to your impression of the benevolent father, I disagree. It's more like a father telling his son that cheerleading is for sissies and that the son has to join the football team.
I wasn't saying all of Demacia was evil or anything. I'm not even saying that Demacia's shade of grey is the same tint as Noxus's, though the D&D reference probably clouded that. Just that they are both grey.