Why were the caliphs killed in ways that contradict Islamic morals?

There is nothing surprising about the deaths of the Rightly Guided Caliphs, for Abu Bakr al-Siddiq (may Allah be pleased with him) was poisoned by Jews, Umar ibn al-Khattab (may Allah be pleased with him) was killed by Abu Lu'lu'ah al-Majusi, Uthman ibn Affan (may Allah be pleased with him) was killed due to the seditions instigated by Abdullah ibn Saba' the Jew, while Ali ibn Abi Talib (may Allah be pleased with him) was killed by Abd al-Rahman ibn Muljam al-Khariji.