How could Leonel not feel the way he did?
When he first entered the Dimensional Verse, he absolutely abhorred killing. He felt that every life was worth the same. He had always thought himself to be a logical person, and because he couldn't deduce the value of a life, he felt that he had no right to dictate the life and death of others. There wasn't any objective measure of a person's life, this had always been his reality.
The first time he entered a Zone, he felt so guilt stricken by what he had done that he almost allowed someone to take his life. No, it wasn't almost, he had already given up at that point. He had already turned 18 by then, and what he thought were his final thoughts were toward his father, telling him to not mind his death because he had already succeeded as a parent, raising him to be an adult.
After he left the Zone, he practically had a mental breakdown after witnessing Aina's sheer disregard for the life that he held so sacred.