If he’s mature enough to lead an army and massacre people, he’s old enough to understand his feelings. Individuals are still responsible for their own actions. A reason why he’s a shitty person should not excuse him from being labeled as a shitty person.
I get your point and I'm not justifying the killings but how will you understand a feeling that you've never experienced yourself in the first place. How will you be able to show it properly when when you don't even know how to. When you grew up in a harsh and violent environment with people only sees you as a monster, a weapon, a tool. Only taught you violence and greed. Well it's just for me, everyone can look at things in a different perspective.
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The one you should all blame here is the emperor. He was the one who fed the ML's mind that he doesn't deserve love nor aim to be loved. My boy was just also a victim of poor guidance and mistreatment from people.