The term "weed" has no formal scientific status at all, including in Ecology. You're conflating how you apparently think of the term with an actual established ecological definition as being an invasive species.
For the rest you're still completely missing the point. It's ethics and morality applied to a specific situation. If I was standing there with a gun to your head because I've decided you're offensive to my garden you wouldn't genuinely believe it's fine for me to kill you simply because you can't do anything to stop it. Likewise if a bigger, stronger person beat your mother or child to death.
You would cry out at the injustice. You would feel I had no right. Your life was your own, and it was horrible for me to end it just because I decided I didn't like how you look. That it was unforgivably wrong for that man to beat your mother or child to death just because he didn't like their face and felt like doing it in the moment. You'd want some authority to intervene and punish the killer or stop me from pulling the trigger. You'd think of them as being the heroes, doing the right thing and saving the day, and me or the killer as the evil wrongdoers.
There's a clear and obvious distinction between taking another life as a necessity because you need to eat in order to survive, and killing on a whim because you decide you don't like how one form of life looks. Even without that, I'd certainly pay money to watch a bear chasing you down and eating you so that I could witness how much you're definitely not talking out your ass when saying that you wouldn't complain about it, because bears needs food.