Why do you think Son Gohan never became an evil Saiyan?

In the movie Dragon Ball Z: The Tree of Might, we learn that Son Goku got a hit on the head when he arrived on Earth. Because of that, he turned out good instead of evil, unlike his other Saiyan relatives. But his DNA didn’t actually change because of it. The Dragon Ball story suggests that the brain injury might have passed on to Son Gohan. Do you think that’s possible?

Why do you think Son Gohan never became an evil Saiyan?
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