No, because in the situation described the rule to prevent harm overrides or takes precedence over the rule to keep promises. No, sometimes one must keep one's promises even if by doing so one permits harm to occur. It all depends on the importance of the promise and the severity of the harm. Those are difficult questions, but in general one needs criteria for judging importance, determining severity, and weighing importance against severity. Without such criteria, one would have no principled way of deciding what to do when rules appear to conflict.
Criteria often come in the form of a theory.
Without both set of rules one's moral system is often either arbitrary or dogmatic. Moral rules and ethical principles are two different criteria. Not exactly, since they don't tell one what to do, that is, they are not action-guiding, morally speaking. These rules tend to be procedural rather than substantive. Each worthy ethical theory is made up of these principles.