"A smile doesn't always mean a person is happy, sometimes.it simply means they are strong enough to face their problems." Jiraiya, Yahiko, and Itachi died qith a smile because they did what they could to help others!
It shows his pain. Sasuke was always kind hearted it was just all his pain dragged him to the darkness and there was no one there to pull him out, but then came Sakura. He loved her since the beginning she pulled him out of his pain and the darkness