What is the ruling on a congregant praying behind an imam whom he does not feel comfortable with, given that there is no hatred or enmity, and is it permissible for him to leave the mosque and go to another mosque?

A Muslim must not hate his Muslim brother without a legitimate Shar'i reason. The Prophet, peace and blessings be upon him, forbade mutual hatred among believers and described hatred as that which shaves away religion. The man mentioned must strive against himself to remove hatred and dislike towards the Imam and other Muslims. It is not permissible to leave the mosque after the Adhan due to disliking the Imam, as the Prophet, peace and blessings be upon him, forbade that.