I am not so sure I agree with your speculation Zamasu was so pure from the start. He was an egocentric prodigy, as incredibly superior in the martial arts than any other apprentice and even confirmed Kais (as Goku told him, the Kaio from his universe wouldn't stand a chance against him), what I think is that his encounter with Goku just fired up his true emotions. Instead of being amazed and impressed a mere mortal could compete with let alone beat him, he resented anger and humiliation. He has a way too high opinion of himself which makes him an arrogant prick lol
Indeed, Zamasu is an arrogant prick, that's what i love about him. But i mean, Vegeta and Present Trunks are also arrogant pricks. Now, we do not know what Zamasu did when he was still on planet Kaishin (talk about "expanded and in-depth backstory"), but i do speculate Zamasu grew up thinking that the Gods were perfect and mortals would just bow before them. People like Zamasu were clearly taught from birth an ideal, and were persuased to believe it for the rest of their life. When Goku defeated him, i imagine that act turned everything Zamasu believed into lies, which probably mentally shattered him. Before meeting Goku, Zamasu was arrogant, yes, but he was willing to realize his mistakes. He clearly needed more training, but he had a pure heart, however clouded by doubt. It's only after losing to Goku that he completely fell into madness.
Basically it's the story of the soul being confronted with their inner demons, and instead of changing his view on humans by meeting one of the most outstanding ones, Goku, he only generated hatred, jealousy and more contempt, which I believe are not the natural characteristics of someone naturally good, like Goku, who immediately congratulates an opponent that can beat him. But Zamasu had trouble hiding his anger, Gowasu is IMO actually the most responsible person in regard as to how Zamasu turned out, almost as if he was in denial, there is indeed many interesting what-if situations we could think of.
Goku never cared about the Sayans. Had he grown up with people around him telling him "Sayans are the strongest, they cannot lose", i am sure Goku too would have thrown a tantrum if an Earthling had defeated him. You also bring up a good point when talking about Gowasu's responasibiliy. Zamasu was his student, it was Gowasu duty as master to teach him the right ways. Gowasu failed, atleast he aknowledges it (towards the end of the arc, he said "This is my sin...Zamasu").
Another important point I would like to outline is I don't believe Zamasu lost his sanity so soon, I think he REALLY turned insane after he escaped the Mafuba, before that he was just his true self, not contained within any kind of structure any longer, and more than convinced by what he deemed as justice. I think all the confrontations he had with mortals before he started to take action only added fuel to the fire of something intrinsically evil lol
Then we are talking about different Zamasus. The Zamasu you refer to is Future Zamasu, which never met Goku, and thus never fell to madness (before meeting Black). Despite being the same individual, Future Zamasu and Present Zamasu/Black have a few, yet very important differences.
