The film chronicles the life of British-born terrorist Omar Saeed Sheikh, who was freed in the exchange for passengers of a hijacked Indian Airlines plane in 1999 and was later accused of killing journalist Daniel Pearl.
Actor Rajkummar Rao plays the titular role in the drama-thriller.
Mehta previously featured Omar in his 2013 "Shahid", in a brief scene where he tries to recruit lawyer Shahid Azmi in his plan while in Tihar jail. Interestingly, Rajkummar played the role of Shahid in that film.
"The guy who was more privileged became this monster, whereas Shahid chose humanity. The irony is, Shahid is dead, he was killed, but Omar is alive. That's the irony of our world," he added.
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He was speaking at the India premiere of the film, here today.
Mehta, who has previously helmed hard hitting films like "Shahid" and "Aligarh", said his aim was not to objectify the life of Omar.
Rajkummar said the character was so intense that it took a toll on him as he tried to get into his psyche.
"I roamed a lot around his house in London but I didn't go to his family and tried talking to them. I'm sure it's emotional for them as well. I stayed in London for three weeks to learn the accent, understand the culture...
Mehta said they would apply for a certificate and said they will "fight it out" if need be with the censor board.
"We are going to try. It has to be within the law of the land. We will do whatever is legally possible," he said when asked about the censor trouble the film might face.