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It was Supermans first battle with supervillains, and the supervillains happened to be highly trained military men, so there's going to be a lot of destruction and fatalities. Superman had to take Zod down asap or there would've been a helluva lot more deaths. Superman seeing that family right in front off him made him fully aware of the carnage him and Zod had just caused the city and he knew the fight couldn't continue, so he made a difficult choice. Plus with all the destruction someone has to rebuild, and I think that's a good way to introduce a certain evil billionaire genious.