i kind of see Shin Godzilla as a primal and carnal being, meaning that it just wants to survive, i believe that he firstly came to the city because he needed to adapt and walked around because (IMO) he desired to know what is the world outside of the ocean is like; he was curious. like any real animal it wants to live without trouble right? if SG won in the film and returned to the ocean, would he destroy other cities to "make sure" he doesn't die? or would be peaceful?
I think the entire drive of Shin Godzilla was to explore and evolve. When he met resistance, he evolved to overcome it. I think conflict with humanity is inevitable.