It's not hard to fit the pieces together. They're easy targets. Humans will pick on them. Take one look at them and go "Stabby stabby" on first sight. Why would a goblin ever be nice to a human if they're treated like that? They will learn to fear. They will learn to hate. They will learn to make targets because that is what they were taught. But just who teaches them?
A popular take, but utterly wrong. Goblins aren't the victims, they are the perps. It's not humans who pick on goblins, it's goblins who target humans. It's not humans who can't live with goblins, it's goblins who rove off of humans. It's like blaming the sheep for shunning the wolf. It's humans who fear goblins, not the other way around.