Going with Sasuke here. He saw his entire clan get slaughtered by the person he look up to the most. On top of that he thought he was the only one left besides Itachi. Plus he didn't even know if it was all an genjutsu or not etc...
Law also saw his family die. Not only that, but Law had to live with seeing all of his friends die as well. He saw children get shot in the face, his entire town. Law saw way more people die than Sasuke did.
He was also treated like a demon for it, with everyone acting Law's mere presence was gonna kill them. Law was essentially Naruto+Sasuke, but all of Konoha was killed and he was there to see it, not just his clan
Sasuke survived because he was allowed to, Law survived solely because he had to hide in a mountain of corpses of his friends and loved ones.
Law's childhood was wayyyy more ****ed up.