Actual Guilt: As a soldier, Rey's killed plenty of people. He also manipulated his best friend, and supported a man who caused a world-wide war to go on for longer than it could've, believing that they were justified.

Mental Guilt:: Whoo boy. Rey feels incredibly guilty over killing Durandal, the aforementioned man, who basically raised him and was almost everything to him. It was for the sake of the world, and he knows that, but he still hated what he did to the point where he decided to die with him. This murder also caused the death of Rey's captain, Talia Gladys, who chose to die with Durandal as well.

Additionally, he's a little unsure of himself, after having been wrong for so long.