Because after that line is crossed it becomes too implausible to identify myself with relate to the other animals. They are too different
Relating to the idea of pain and suffering is implausible

Which means that you have not understood what I was trying to say. Whether this is deliberate or something on my part I don't know.
That's what it comes down to. It has the genetic make up of a human diploid cell. That's what decides whether killing it is ethical or not.