Superman was prepared to kill by the end of OWAW, though he didn't really end up having to do so.
The problem with the no-kill code is it is unrealistic and given a sufficient level of realism, it isn't even that moral. However, typically we need to accept that at typical levels of comic realism, it is meant to signify his superior morality. But sometimes writers make things more realistic and fuck around with how moral it is. But those tend to be atypical. Ditto for Batman's code, including his no guns code (which is super silly).