Why were the early Caliphs killed in ways that are unfitting for Islam's elevated human morality?

It is not expected that those who strove to eliminate the Rightly Guided Caliphs would adhere to noble character. Abu Bakr was poisoned by the Jews, Omar was stabbed by a Majusi (Zoroastrian), Uthman was killed by a conspiracy orchestrated by the Jew Abdullah ibn Saba and some wicked individuals, and Ali was killed by the Kharijite Abd al-Rahman ibn Muljam. These people have no morals. Allah honored these virtuous individuals with martyrdom.