Were all messengers of God preserved from murder, because they are the best of humanity and greater than the prophets?

What you mentioned, that the Quran did not state that some prophets were killed, is incorrect. Allah Almighty informed us that the Jews killed some of His messengers, such as Zakariya and Yahya (peace be upon them). Allah's allowing disbelievers to harm some of His prophets is not due to any deficiency in their status, but rather for profound wisdoms; such as elevating their ranks, for them to be an example in patience, and so that no one would excessively revere them.