This question can be from a person who did not understand the difference between Shia Muslims and Sunni Muslims. Shia Muslims believe that the divine leadership of Islam after the Prophet Muhammad (SAWA) is for Imam Ali (AS) and Ahlul Bayt (AS), according to the orders of Allah and the Prophet,  while Sunni Muslims follow Abu Bakr and Umar and Uthman despite claiming that they respect Ahlul Bayt. Most of Sunni Muslims respect Mo'awiyah ibn Abi Sofyan, whose son Yazeed ibn Mo'awiyah killed Imam Hussain (AS) by clear orders from him to his governor of Kufa and his army.

Imam Hussain (AS) never believed in Abu Bakr as Khaleefa nor in Umar and Uthman and never gave them allegiance as Khaleefa.

imam Hussain (AS) stood against and was killed by Yazeed who was appointed by his father Mo'awiyah, who was originally appointed as a ruler on Syria by Umar ibn Al-Khattab.

Claiming that Imam Hussain (AS) was a Sunni is like claiming that Imam Ali (AS) was a follower of Mo'awiyah.

Such claims go against the clear facts and have no value.

Wassalam.