Entirely depends on the character, and why they killed the person. 

In complete self-defense, a brief mixture of guilt and triumph. 

As an accident, large portions of worry at first, but then switching to guilt and fear. 

In revenge, there would be some satisfaction at first, followed by a cold empty feeling. 

In cold blood, very little would be felt other then some insane rantings within the mind. Perhaps some mild satisfaction as well. 

In an uncontrolled rampage, extreme guilt.