Have you considered that maybe God, who is love according to the Bible, designed this universe to be a complete demonstration of love? How can you fully demonstrate love if you don't show what it means to love someone who's evil and considers you an enemy, or someone who doesn't even believe you exist, or someone who once thought they knew you but were being deceived by people with evil motives?
If God created this universe as a demonstration of love, then why the fuck is there sections of the book where he wipes out entire families with disasters because he got angry?
I have considered that. There is a lot of evil (or suffering) that nobody directly causes and especially not because they're evil. Why is there depression for example? Or cancer?
As for where it came from, it was all brought about with Adam and Eve's first sin, which infected all of creation with decay. You could write a creepypasta about that. Depression's a bit more complicated because it's a thing in the mind, and there's a case to be made that it's often more directly a symptom of a separation from God, knowing on some level that something's missing - but I don't think that can be said of all depression. Either way, it still ultimately stems from the first sin.
As for why it should exist for a time, it's again necessary to be able to demonstrate love in those circumstances. It's easy to love someone who's always having a good time, but it's divine to see your love and support help to pull someone out of depression, or to comfort someone who knows they don't have long to live. (This isn't just about the love God pours out, but also the love He inspires in His people.)
Why would a loving god punish unrelated people thousands of years later with cancer and such for a harmless sin that he must have known Adam and Eve would commit?
Why should sickness exist at all? Why can people die when killed? Why do bad things happen to good people? Why isn't life like Heaven for people who haven't done anything wrong?
That's the way it originally was, but sin messes things up. And it's in our very nature. Nobody can say they've done nothing wrong because everyone has at least one concrete sin they've committed, often several.
There's no such thing as a "harmless sin." It has to be black-and-white or else people will try to weasel themselves out of accountability ("I'm only lying, it's not like I'm killing anyone.") Similarly, you can't good-person your way out of being a sinner. Even if you could stop sinning all on your own, if you're not going to repent for what you've done, the guilt is still there.
...Also, as previously stated, you can't fully demonstrate love if you don't show what love is like in hardship.