I felt like they did need to explain it
I just don't see how it would've hurt anything to tell us at least what was causing it (even if they never showed us an actual "monster"). B/c there were just way more questions than answers. Besides what was causing it we also had the why were there some people who immediately were suicidal but others could see and live.
I get that they stuck to the book but it wouldn't be the first time that a movie took a different path from the book, especially for those of us who never read the book.
I'm just not someone who likes to be left with a mystery with no resolution. Unless they're planning a sequel (which I could see them doing).