There is actually no black and white, but shades of dark. Let me post a chronology of events that led to Mirjafars betrayal.
And oh, by the way, Nawabs of Bengal were Turks. not Pashtun.
Jagat seth is a family title enjoyed by two brothers. I did read some research papers on this period and now I would request you to remove some misconceptions that we inherited from our school text books that portayed Siraj as great independant Nawab.
[1] There was seven Hindu Kings/Zamindar under Nawab of Bengal: King of Nadia Krishna Chandra ( who was main conspirator), King of Burdwan, Kingdom of Natore, Kingdom of Ban Bishnupur, etc....
Nawab never ruled Bengal directly. These local kings were ruling more or less independantly paying taxes to Nawab. Nawab's army was also highly dependant on the supplies from these seven kingdoms.
In the first war against British (1748), King of Burwan supplied all the logistics to 40,000 army of Siraj.
[2] During the time of Nawab Alibardi ( grand father of Siraj), relationship between Hindu Kings and Nawab was very cordial. Attack by the Maratha invadors (Bargi) was common ground to be united. While fighting against one of such invasion against Bargi, Alibardi was running away after losing a battle in Bihar. He was abondoned by all of his Generals. However, one small group leader (Mansabdar) didn't leave him. This man was Mirjafar. Strange as it sounds, upon return to Murshidabad, Alibardi elevated Mirjaffor to be one of his main General, as a reward to his loyality and faithfulness. Mirjaffor was a devoted Muslim. He was very loyal and honest to Alibardi. Later on, he was married to Alibardi's family.
[3] Nature of Siraj's communalism needs to be adjudged against [1] & [2]. Several times Siraj expressed dissatisfaction that he had to depend on Hindues for any kind of financing and he predicted that these Kafirs would betray him one day ( he was right indeed). Besides, his force abduction of any woman was a matter of great concern among the Hindu Kings as he even didn't spare their families.
He abducted daughter-in-law (not daughter) of older Jagat Seth brother ( Manik Chand probably). However, he was vehemently opposed by Mirjaffor who was a man of moral character (!) and finally he had to abondon the abduction. Maharani of Natore had a beutiful widow daughter. When she was visiting her palace in Murshidabad, Siraj got a glimpse of her daughter's beauty. He immediately sent his army to capture her. However, the news was leaked and Maharani's widow daughter was evacuated in the disguise of a dead woman.
The fact that Alibardi was a man of honor and Siraj was communal is revealed through several documents:
[A] Communication between Krishna Chandra and Jagat Seth brothers. From their letters, you would learn that Jagar Seth never wanted to help British against Siraj. Reason is simple: Jagat Seth's main business was financing and banking. They knew once East India company would get their grip, they will be knocked out from the business. ( Their business was indeed ruined by East India company). They knew that their business is always safe under Nawab.
However after the abduction of their daughter-in-law, they had no choice but to agree to King of Nadia,Krishna Chandra, who was indeed the first and core conspirator against Siraj.
B. Communication between Lord Clive and East India company senate: Clive mentioned in his communication that Hindu kings are extremely hostile against Siraj and they would help in any attempt to depose him. Clive mentioned that Hindu kings are greatly worried about to protect their religion under Nawab's rule.
C. Communication between Maharani of Natore and Maharaja Krishna Chandra
D. Communication between King of Burdwan and Krishna Chandra:
In C and D, you will find explicit language used by Krishna Chandra to term Siraj as communal. His letter indeed started as an appeal to protect Sanatan Hindu religion against Siraj.
One has to remember, Krishna Chandra and Alibardi were great friend. Krishna Chandra was very loyal to Muslim Nawab Alibardi. So we can not term him to be born communal.
[5] Siraj had nearly 50,000 soldiers against 3000 British soldiers! So he had nothing to be afraid of the British! Hmm..But he was afraid. He was afraid of being betrayed. He knew that Hindu kings were no more on his side. Lord Clive indeed rested in the palace of Krishna Chandra on his way to Palassey. Krishna Chandra provided local logistics to Clive.
Siraj insulted Mirjaffor times and again in the Royal court. Madanlal and Mohanlal, these two Kashmiri hindu generals were his favorite,as myth revealed. He knew Mirjaffor might betray. But still he never trusted Mirmodan or Mohanlal to be in charge of Royal army. They were in charge of a small troup of 1500.
Siraj-ud-Doula: Clarification from history* Biplab Pal