A "killer feature" is a phrase with its own meaning so we can treat it as an atom and apply modifiers to it. The order of operations matters, and I posit that the OP placed their imaginary parentheses as I did in my first example.
No. There is no such thing as a "literally killer feature" unless it is some feature that can cause death. A killer feature in this case is exclusively figurative and adding "literally" is wrong to do.
No - it is figuratively the killer feature.
If it were literally the killer function this would be a very different discussion.