Trauma can beget evil pretty easily, and trauma can be caused by non-sentient sources.
... So I feel like this is a HUGELY problematic statement, and I'm not equipped to unpack that so let's just throw out the suitcase.
Man helps build a bridge. Man makes earnest mistakes building the bridge. Bridge collapses and wipes out half of a family. Survivor of the family wants justice but the system respects that this was a mistake and gives the bridge-builder a hefty fine. Man who lost half his family spirals due to having little support in his small peasant community. Begins to have dark thoughts. Eventually acts on dark thoughts.
So let's touch on this one:
1) Why is there a fine? What good does a fine do for anyone? What good does punishing the builder do? Or is it just because we've been raised to expect punishment for misdeeds and mistakes?
2) Why doesn't he have support? It's a community of good people. That's altruistic, kind, caring. People support each other in the modern day when there is a loss. Why would that not happen in a world where only good exists? In a society that is good, people would see him struggling, and seek to get him the support he needs (Whether they can personally provide it or help him get professional support).
3) Why doesn't the good man who lost his family see that it was an accident and thus place blame? Is it because we're culturally pre-disposed to seeking someone to blame? In Spanish "Timmy broke the table" is a grammatically weird sentence and "The table broke" is a grammatically normal one. Would we have the same expectation of blame if our culture didn't value revenge?
This may seem implausible, but C-PTSD can cause some people to veer towards antisocial behaviors. Trauma can also trigger other disorders, like Bipolar, which can lead to manic episodes wherein the sufferer does out-of-character things that can easily be perceived as evil or, at the very least, chaotic or harmful. Likewise, drug addition can happen even when prescribed rugs like opiates -- milk of the poppy in ancient times -- and can cause people to resort to pretty loathsome behaviors as the addiction twists and turns them.
Now imagine a society where people with C-PTSD, and all other forms of PTSD, and all other neurodivergences, get the love and support they need to heal, instead of a world of goodness where somehow this one example of casual callous cruelty is just allowed to fester.
We can par back all the realistic stuff and go pure Fantasy, saying: man's child dies. He can't accept it. He'd do anything to have them back. They were his rock, his everything. Over time, he spirals and turns to black magic to try and ease the pain of the loss.
What black magic? It's gone. All the evil is gone.
This is like saying "In a world where there are no guns, a man shoots his neighbor with a Remington 30/30"
Everyone in the world, including the gods, can be universally good, but tragedies and misfortune still happen, and tragedies make more villains then evil gods do. That's because pain + human creativity and problem solving ability = dark "evil" things being reinvented.
Tragedies happen. But not every tragedy leads to people committing war crimes.
There's a lot of off-ramps between A and Z that make it a -very- bumpy slope, rather than a slippery one.