Article 5 is subtle. Consider an issue that is controversial, but equally so in every state. 51% for 49% against in all 50 states. It sails through the ratification process without difficulty. Contrast that is an issue that is geographically divisive. Approved in some states, rejected in others. An amendment on such a topic could be blocked by 25% of the states. If those are low population states, it could be blocked by well under 25% of the population. I think that is genius. Amendments that don't set state against state are easy. Amendments that do set state against state are hard. Just what you need to hold the country together.

should certain states be allowed to make murder legal?

To the extent that the case is obvious, the State and Federal levels should agree. No need for federalization. But then we get to tricky edge cases. Was it murder or self-defense? Was it murder or just a citizens arrest gone wrong? Federalization looks really bad here. It puts an end to the business of saying: that other state does it better, we should copy. Notice what happens when a federal reform goes bad. People just say: times changed, it wasn't that the reform made things worse. You cannot cheat like that when a state level reform goes bad, because other states don't suffer from "times change".