It's not odd that it created the Zimbra account, it needs that to run as a 'user' on your system that's what all applications do - there's nothing suspicious about it.
You should have removed Zimbra by running the install script again with the -u option i.e. './install.sh -u' - that would completely remove it. This is all documented in the quick start guide. As I said I'm not a MAC user so I don't know how to remove a user account from the machine but a quick google found these
instructions. try one of those examples and see how you get on.