At his core Zamasu is good like Shin, though I guess you could say he was so fastidious that it backfired. But you know, for this "Future Trunks Arc" you had to depict Zamasu and Trunks' inner conflict, right? If this was back when I was drawing the manga myself then I doubt if I could have done it. I mean, I'm not very good at depicting the characters' psychology on the page. So this all came together because now I only have to think up the story. [...] On my own, I doubt I would have been able to express Zamasu's fall to the dark side.
Akira Toriyama, DBS vol.4 joint interview with Toyotaro