These positive consequences for the Imam (A.S) and his followers made Ma'mun understand that he had made a big mistake, had to solve this problem and found no way but killing the Imam (A.S).
A) He could disclose his opposition with the Imam (A.S), which had two consequences: 1) No one protested against him and could control the situation, which endangered all Imam (A.S)'s followers. The people such as Safvan-ibn – i-Yahya, Muhammad ibn – iAbi Amir and etc. who were Imams' trainers in his time and the narrators of Imams' beliefs for next generations and the writers of many books and dissertations in many different topics about theology, history and Shi'a traditions were endangered.
It might lead Imam's (A.S) followers and every enemy of Ma'mun to take advantage of Imam's killing and might commit practical objections and armed uprising, which resulted in cluttering the base of government, ensuing chaos, and providing grounds for falling the central government, endangering atheists' entrance, out of Islamic borders, and their flowing for demonizing Muslims.
B) He killed the Imam (A.S) secretly and pretended that the reason of Imam's (A.S) death is because of illness, which occurred suddenly. Consequently, Ma'mun would remain Imam's (A.S) fan.
Ma'mun himself was more interested in killing secretly since he was scared of the consequences out of killing the Imam (A.S) openly. However, if he had had to commit such a crime, he would have certainly done it.
Therefore, Ma'mun followed the second plan. He invited the Imam (A.S) to a consulted meeting. After they talked for some moments, he brought some poisonous pomegranates and grapes for the Imam (A.S) and requested him to eat them.
The Imam (A.S) had two ways: He could avoid eating poisonous pomegranates and grapes, which meant that Ma'mun had to kill the Imam (A.S) openly and announced his open opposition with the Imam (A.S) and his followers. Another way was to accept eating them and to make Ma'mun continue his politics – pretending to be the friend of the Imam (A.S) and his followers in order that Shi'a organization to be protected from Ma'mun's attack. The Imam (A.S) recognized the second way as a good way, so the entire attempt of the Imam (A.S) was that Ma'mun did not confront the Imam (A.S) and his followers openly and formally and he had to kill Shi'a people by swords.