Curze knew what he had to become to save his world, his instincts were to kill and torture from the moment he landed, once he became aware, he was aware he needed to make people fear him, it was built in to him, the same way Guilliman had a knack for being a statesman or Sanguinius was inspiring and beautiful. It's becoming increasingly obvious that The Emperor had a plan for each of his sons, a role they were destined to play, he made Konrad a monster because he needed one, Angron was a brute because the Great Crusade needed brutes as well as generals.

Those two were broken and pitiful because The Emperor needed them to be, he didn't care about the damage that would do to them because the ends always justify the means to someone like the Emp.