I am not an antinatalist, I do not believe that human life, broadly speaking, has no value, in fact my position is that human life does have value - hence why some human life doesn't.
There's a Latin expression: "homo homini lupus" - man is a wolf to other men. Vonnegut once wrote 'there are plenty of reasons to fight, but no good reasons to hate'. If, say, there are humans committing genocide against other humans, those humans committing genocide are, in a humanitarian sense, value negative.
I would take it much further. Though I am not interested in actually going out and killing humans, it's simply not within me to be able to do such a thing, preference utilitarianism is very salient to me, but others seem indifferent to its conclusions. I see it as the only ethical system that could possibly be said to be "objective" since, afterall, if morals are just a subjective set of preferences, then the 'objective' (intersubjective, whatever) morality must be the one that prioritises, and seeks to maximise, preferences. QED.
So long as people are convinced, without good reason, that their own totally subjective, often obscene, moral codes are the capital-T "Truth", I see no good reason to value their lives. In other words; if you don't value the life of others, then there's no compelling reason for me to value yours. I certainly wouldn't kill you, but I would argue that your life has no value. If you are ignorant and evil, you are simply not worth my moral concern. It sounds callous, I know, but the important thing to recognise is that it's callousness only in response to the callousness of others. Pretty simple stuff, at least as far as I'm concerned.
To put a little pithy summary on this: there are some people in this world who think that the millions of children who routinely starve to death somehow deserve it when, by virtue of holding this perspective, it is actually they who deserve to die. Maybe not of starvation, but their perspective makes them useless, even a drawback, to the rest of humanity.
As for the second part of your question: this appears to be pretty normal, albeit reductive and naive, moral reasoning. When I was still a fist year I learned it as "The Sanctity of Life", it's especially apposite in Western European Cultures*, and it means there's some debate about whether a mentally lucid fifty year old with fibrodysplasia ossificans progressiva (the disease that ossifies lean tissue - turns your muscles into bone) should be able to choose to die. It's clear to me that this kind of thinking is just another example of how the philosophy of a compassionate Judean anarchist can be deformed by knaves into something that is, frankly, sadomasochistic.
*Eastern Europe is different, apparently. According to Slovenian philosopher, Zizek, they have a joke: "A man is cleaning his house one day and comes across a magical lamp. As is the way with these things, he rubs the lamp and a genie pops out. The genie tells the man he can have anything he desires, but he also warns him that whatever he wishes for, his neighbour will get twice that. The man thinks on this for a moment and finally says: "beat me half to death."