I had something similar happen to me when I was about 4 years old. My father would kill my pets in front of me. Once my cat had kittens so my dad took them, put them inside a burlap sack, and hit them with a ball-peen hammer until they stopped moving. Another time he drowned them in the creek. The implication was that I was next.
I did not understand the concept of death at that age, but I knew something was very wrong. It made me feel sick inside and I understood enough to know he was hurting them. I was never forced to kill any of them, but I am sure I would have felt complete disgust if I had been just as you have and that shows me you are a healthy, loving person who cares about animals.
I am sure it stills seems like you did something wrong, but you didn't!!! You were totally innocent.