The problem with unbreakable rules is that you're only allowed to have one. Suppose I have a moral duty to tell the truth no matter what and a moral duty to protect the innocent no matter what. Then what do I do if I find myself in a situation where the only way I can protect the innocent is by lying?

More generally, real life finds us in situations where we are forced to make tradeoffs, and furthermore, real life is continuous in a way that is not well-captured by qualitative rules. What if I think I have a 98% chance of protecting the innocent by lying?---or a 51% chance, or a 40% chance? What if I think a statement is 60% probable but I assert it confidently; is that a "lie"? &c., &c.

"Lying is wrong because I swore an oath to be honest" or "Lying is wrong because people have a right to the truth" may be good summaries of more-or-less what you're trying to do and why, but they're far too brittle to be your actual decision process. Real life has implementation details, and the implementation details are not made out of English sentences.