Ok, so to answer a few things:
To you question about plants: If plants had Human-level intelligence, and human moral capacity, then yes we could not eat them. If your only food source is a person, your only valid choices are to die or find some alternative. Anything else makes you a monster. But plants dont have human-level intellect or persona, so it is not relevant really.
Please don't dodge by saying that "it's not real so it doesn't matter." None of this is real. I'm asking you if you're stating that in a clear case of predator/prey in which one MUST kill the other to survive, that it is the predator that must be exterminated. That instead of enduring the occasional death of members of one species, that it is preferable to go with mass genocide. And if you would willingly and happily consign yourself to the same fate if you found homogeneous sapiens in that boat.
Besides, it's a big universe. You literally don't know if the same thing cannot be said of us. If dolphins are discovered to be sentient and mankind is killing them by polluting the oceans and causing climate change, does that mean that if man won't give up industrialization, we must be exterminated?
As for Liverspots: Who cares if he was a person? I have never understood why anyone finds this significant. Hitler was a person. What of it? Your life no longer has ANY value if you commit acts that nullify its value.
Michael would disagree with you. Nicodemus is arguably a worse person than Hitler, having done more damage over his thousands of years and committing more atrocities. And yet Michael risked his life, Harry's life, Uriel's grace, to offer the chance of redemption. No one has gone so far as to not turn back.
If you disagree, OK, that's your opinion. Dresden - at least, Dresden as he is now, and certainly early Dresden - would agree with you. But just because you say that he's a monster and should be wiped out without a second thought doesn't make it gospel truth.