"Someone who hates the whole world... " should also be acceptable
It does have the same meaning, but that would be more like מישהו ששונא את כל העולם...
He who hates?
אני שונא חלקים מסוימים של העולם אז אני מופרע רק קצת בצד שמאל. טוב לשמוא! :-)
In this case, what's the difference between "anyone" and "someone"?
Well, I think anyone stresses more that any person whatever who fits the description is affected. maybe you could express this by כׇּל אֶחָד שֶׂ־, but Hebrew does not clearly distingish between someone and anyone.
"Hebrew does not cleary distinguish between someone and anyone." -that was the point