The Umayyad murdered the Imam, peace be on him. The reasons for that are as follows:
1. The Imam's high Character
Imam Mohammad Baqir(A.S), had the highest character in the Islamic world. Moslems have unanimously greed on his high character. They have admitted his outstanding merits. All Moslem religious scholars from all Moslem countries came to him to learn his sciences and his morals.
For the Imam learned that from his grandfather, Allah's Apostle, may Allah bless him and his family.
The Imam, peace be on him, possessed the sentiments of the people. Thus, they admired and respected him.
For the Imam was the most prominent figure in the family of the Prophet, may
Allah bless him and his family.
Hence, the Imam's high social rank displeased the Umayyad. Accordingly, they decided to murder him to get rid of him.
2. The Events of Damascus
Among the reasons that moved the Umayyad to kill the Imam, were the events the Imam faced in Damascus. These events are as follow:
A. The Imam exceeded the Umayyad and others in shooting. That was when Hisha`m summoned the Imam to compete with the Umayyad and others for shooting. Hisha`m thought the Imam would fail in hitting the target. Thus, he would be able to use the Imam's failure as means to degrade the importance of the Imam and to mock him before the Syrians.
However, the Imam shot and hit the target several times. The people had never seen such skillfulness in the world of shooting. Hisha`m was full of anger. So, he decided to murder the Ima`m from that moment.
B. The Imam debated with Hisha`m on the affairs of the Imamate. The Imam defeated Hisha`m. So, the latter was full of malice towards the former.
C. The Imam debated with the scholar of the Christians on certain affairs. The Imam also defeated the scholar. So, the Syrians were busy talking about the Imam's victory over the Christian scholar.
source : www.tebyan.net