Are Kharijis Muslim? Will they ever go to jannah? Why or why not?

The Khawaarij were not declared apostates by the Sahaabah (radiyallaahu anhum) when they revolted and waged war against the Muslims. Rather, they were considered heretics and rebels.

However, certain sub-sects appeared later on whose disbelief the scholars did not differ on, such as the Bid‘iyyah who claimed that salaah is just one rak‘ah in the morning and evening, the Maimooniyyah who made marriage to certain mahrams lawful and denied that Soorah Yoosuf was part of the Qur’an and the Yazeediyyah (not the religion) who claimed that Allah will send a messenger among the non-Arabs with a new law abrogating the sharee‘ah of Prophet Muhammad (ﷺ).

And Allah knows best.