I don't think it is a rule that it is going to warp one's vision, but I've seen people getting depressed and definitely biased when studying philosophical pessimism. It seems like something that only happens in jokes or memes, but no, reading Arthur Schopenhauer or whoever can be dangerous if one is already vulnerable to depression, isolation, etc.

I definitely advise discretion. And it's not because they're dark monsters, monks of death dressed in black robes. There's nothing too morbid about the books; that's probably just the myth time has created around them. In reality, their danger is just pondering on dark aspects of life that can be disheartening if one is not prepared. Even when the reading is for high school or university, or for curiosity, I think these authors should be picked with an open mind and a serene "heart".

Thank you for reading and answering.