If you believe all people have at least some basic measure of worth or value, why do you believe that?
Interconnectedness of everything is one reason. I'm a non-materialist monist. Probably even a panentheist, as I understand Buddhas to reflect Nirvana actually. How could anyone say another person is less than them, or justify their suffering not to matter?
I extend this to animals also because they are sentient. Jains have a concept concerning karma that I'm rather favorable to. That plants and rocks are forms where karma is fixed. It's not that they are not life as well. They are inert enough that real suffering cannot be inflicted on them like on people and animals, but they also cannot awaken.
Given the pain of a world that suffers so much, this unity of all things is what can make life seem beautiful in spite of pain. Knowing I am not really separate from plants, oceans, rocks- it's an awesome and uplifting thought.
All life seems worthy of existing, and violence seems a great evil. It's almost like you're violating everything when you harm.
I cannot verbalize entirely how what I've said to you makes me feel. The great wonder and magic of it all. That's exactly how some teachers described life in total. That it should exist at all seems magical. The universe might be a magical display.