There might be several more reasons for the Umayyads to bear malice towards the Ahlul Bayt (a.s), but for the Abbasids, who came to power on the slogan that the caliphate was the inherent right of the Ahlul Bayt (a.s), animosity could spring only from a desire to cling to the power that so fortuitously fell in their lap. But a more important reason was the suspicion of an imagined threat from the Ahlul Bayt (a.s).
However, the surprising thing is that the Ottomans, Ghaznawids, Mongols, and other Muslim rulers all over the globe, such as Saddam in recent days, bore animosity towards the Ahlul Bayt (a.s) in general and the Shia in particular that can not be normally explained.
The root cause is to be found in the following facts:
Immediately after the Prophet’s demise, several legends were invented to create a divide between the Ahlul Bayt (a.s) on the one hand and the rest of Muslims on the other. Firstly, a tradition was put forward as an argument against the claim of the Ahlul Bayt (a.s) that the Prophet (s) had said, “We the prophets neither inherit nor bequeath.” The tradition appeared to be so noble in content that it was accepted by many without inquiring whether the Prophet (s) had really said so or not. Obviously, the said tradition is contrary to the Qur’an which speaks about Prophets inheriting one from the other and Prophets praying for a successor to carry on the Divine Mission. Hence, the tradition is an obvious invention. In fact, none from the large number of the Prophet’s companions, except two persons, testified to hearing the said tradition from the Prophet (s).
Another legend was that the Prophet (s) did not wish to place the spiritual as well as the temporal leadership in one place. Even this tradition had no corroborators. No reason was given as to when and why the Prophet (s) said so, particularly when he himself held both the offices. This tradition is also contrary to the Qur’an that speaks of the kingdoms bestowed upon the Prophets David, Solomon…etc.
Regarding the invention of such legendary traditions, Nicholson wrote, “During the first century of Islam, the forging of Traditions became a recognized political and religious weapon, of which all parties availed themselves. Even men of the strictest piety practised this species of fraud, and maintained that the end justified the means.”
The effect of the legends was that the infallible Imams became the acknowledged spiritual leaders while the Caliph assumed the temporal rulership. The only object of these legends was to separate the spiritual
 A Literary History of the Arabs, p. 145 .
leadership (wilaya) from the temporal rulership (mulk), and to keep the temporal leadership out of the reach of the Ahlul Bayt. It is this later motive that was responsible for the creation of further fast legends such as that Ali and his Shia never offered prayers and that the Shia were heretics. The false propaganda that Ali and his Shia were heretics deserving to be cursed after every prayer, was first started by Mu’awiya in the year 12 AH, and spread by him throughout the Muslim world that he later came to preside over. As a result of Mu’awiya’s orders, Imam Ali (a.s) and his Shia were cursed from on seventy thousand pulpits everyday, and false stories about them were spread throughout the Muslim world for over half a century. This put unshakable roots in the minds of common Muslims, so much so that even in these enlightened days they persist in several Muslim countries.
Sheikh Shamsuddin Abu Abdullah bin Makki bin Hamid al-Aamuli al-Juzaini known as ‘the First Martyr’ was a great scholar of his time. He had written several books. The accusation against him was that he was a Shia and therefore deserved to be killed. First, he was imprisoned for one year and then he was asked to tender an apology that he refused because it would then amount to admission of guilt. He was martyred at the instance of Judge Burhanuddin al-Maliki and Judge Abbaad ibn Jama’a ash-Shafi’iy on Jumada II, 786 AH in Damascus. On the persistence of Judge Abbaad ibn Jama’a, he was beheaded and his body was hung from gallows and later was burnt.
The ‘Second Martyr’ is Sheikh Zainuddin bin Ali bin Ahmed bin Muhammad bin Jamal bin Taqiyyuddin bin Salih. He was martyred on the allegation that he was Shia and so he deserved to be killed. When he came to know that he was likely to be arrested, he left on pilgrimage. When the Judge came to know this, he wrote to the king of Rome that a person who was not from the four sects of Sunnis and who was a Kharijite had taken refuge in his (the king) domain, and that he should be arrested forthwith. The king sent an emissary to find out if what the judge had written was true, and at any rate to apprehend the man and bring him alive. The emissary found Sheikh Zainuddin bin Ali in Mecca. The Sheikh asked to be his guest until he would complete the pilgrimage. They both left for Rome. On the way, they met another person. On being told that the Sheikh was a Shia Scholar, the stranger told the emissary, “Do you not apprehend that this Sheikh may complain to the king that you have ill-treated him and that on such complaint the king
may punish you?” The emissary believed that such could be the case. He beheaded the Sheikh near a canal. He carried the head and left the body. During the night, the residents of the village saw radiant beings visiting the spot where the Sheik’s headless body was lying. The next day, they buried the Sheik’s body and built a building with a dome over it. This happened in the year 966 AH. When the king was enraged to see that his emissary had killed the Sheikh and brought his head instead of following his orders, the emissary was hanged.
 Shahide Salis (the Third Martyr) by Mirza Mohammad Hadi Sahib Aziz Lucknowi, p. 12-13.