A parallel between Kenshin Himura and Erwin Smith

Kenshin Himura, a former war-assassin who had killed and stolen so many lives, held a sense of deep regret and guilt, hence he thought nothing of his own life and disregarded the fact that his, too, is another human life. Because he had slain so many lives, he didn't even value his own and was more than ready to throw his life away in battles.

Here we see a parallel in Erwin Smith from Attack On Titan:

He was burdened by guilt for "leading soldiers into death", believed that everything he did was just for his personal search of truth, hence he did not value his own life - as apparent in his talk with Levi before the Shiganshina battle. He thought that his life was "expendable" and that anyone could replace him. He never realized his importance to the Wall Society's survival and that his talents and skills cannot be easily replicated.

However, Kenshin had a mentor/father figure who taught him the importance of valuing his own life, and he grew stronger from that. Erwin doesn't.