Evil is the intention or deliberate action to cause harm. what is harm? Well you can define that on principles or make up a laundry list, neither of which may be complete or accurate. However, the difficulty in producing a robust definition of harm does not detract from this robust definition of evil.
True. Could you be said to cause harm by failing to act?
Guilt can be felt by simply making a mistake. A doctor doesn't intend to do harm if he amputates the wrong leg, but he might feel guilt about it. What he or she feels is beyond the grief anybody else would feel about the matter. It's changes from grief to guilt because he took responsibility for the patient.
So the more responsibility you take on, the more subject you are to guilt. Honor and duty are complex ideas. The harm involved extends out beyond one event. You see how your actions impact your world.
But I imagine the origin of guilt is in childhood. The roots being empathy and the withdrawal of fondness from others.