Also, if you do something every day, it is not an exception.

This is simplifying the definition to the point of defining the term out of existence. No one actually has any convictions in this world. This is actually kinda bad if your goal is for people to really think about ethical issues and try to maintain a degree of rational consistency.

Plus being so black and white in the manner you're describing would.. well actually be really stupid a lot of the times. The fact that Batman doesn't kill the Joker is a storytelling device, in the real world it would be monumentally stupid to do anything other than blow his brains out. Literally millions of lives saved. But it also makes sense, and his good, that Batman still maintain is strong conviction to not kill despite choosing to do it sometimes.

Rules necessarily have exceptions and it's healthy to do so, black and white thinking should be for the jedi/sith, not real life humans.