pkg: /data/local/tmp/Superuser.apk Failure [INSTALL_FAILED_ALREADY_EXISTS]
The problem isn't that a file already exists at that path: it's that the app you're trying to install is already installed on the phone. Have you previously tried to install SuperUser as a normal app? You need to uninstall it before you can root the phone. Uninstall it in the usual way from Settings → Apps.

In addition, the Motochopper exploit only works up to the 4.2.2 firmware for the Galaxy S4 (and only S4 variants with the Snapdragon chipset, not Exynos-based ones). If you've already updated to 4.3 or 4.4, this exploit can't work. You need to use another method, for example unlocking the bootloader the conventional way (though if the phone is carrier-branded the carrier probably has disabled unlocking the bootloader).