Sorry to hear about your frustration with the plugin. I totally get how that might be confusing as to why data is left behind after deactivation and/or uninstallation. We would never remove data if a plugin was deactivated - you can imagine the frustration a user might have if their data disappeared after deactivating the plugin then reactivating it.
Uninstallation is a little more difficult - There are a lot of reasons a plugin might get uninstalled - often times, users will uninstall a plugin to try to debug other issues on the site, intending to reinstall it after debugging. For this reason, we choose to err on the side of caution, maintaining the integrity of user data.
That said, though your database had data from the plugin in it - I'd be surprised if that data was responsible for 'screwing up' your database.