***** NOT A TEACHER *****
Hello,
I know how confusing this can be. Even native speakers often use the "wrong" form.
1. Many native speakers go to a movie and then say, "Wow! That's the best movie I ever saw in my life."
If you think about that sentence, you will realize that the speaker does not expect to see another movie
for the rest of his life. Let's say that he has gone blind. He could then use the past, for -- sadly -- he is
not going to see another movie ever again (of course, he can listen to it).
2. So most people should say, "Wow! That's the best movie that I have seen up to this point in my life.
But who knows? Chances are that you will see another movie next week and then yell, "Wow! That's the best
movie that I have ever seen (so far)."
3. So if you tell some native speakers "I did not kill a single animal in my life," they will smile and ask you
jokingly "Are you a ghost?" So you should say "I have not killed a single animal (up to this moment in my life)."
But nobody knows the future. It is possible that sometime in the future you might kill an animal. Let's say
there is a terrible war. There is no food available. So you might have to kill an animal in order to feed your
family.
James