One person being harmed isn’t always a benefit to one or more other people.
Though, really, seeing as utility between persons isn’t clearly commensurable, it isn’t clear to me that the statement that the world is zero sum even, like, makes sense as a claim, because there’s no “sum of everyone’s utility” to ask whether this sum is constant.
But, if there was such a sum, it wouldn’t be constant.
We only have one Earth for now.