Brigham Young called her a devil because she opposed him as Joseph's successor even though she tried to protect the original instructions the way Joseph Smith had taught them.

Whether or not Emma liked it Young was now the second President of the LDS church and by disobeying and openly disagreeing with the prophet Brigham, the Apostles, and some of church teachings she was now an apostate.

[Church Handbook of Instructions, Book 1, Stake Presidencies and Bishoprics, 2006, Page 110 says under the section Apostasy:

Repeatedly act in clear, open, and deliberate public opposition to the Church or its leaders.

Persist in teaching as Church doctrine after they have been corrected by their bishop or a higher authority.

Continue to follow the teachings of apostate sects after being corrected by their bishop or a higher authority.

Formally join another church.

In such cases, excommunication may be the necessary if repentance is not evident after counseling and encouragement. Priesthood leaders must take disciplinary action against apostates, to protect Church members.]

Emma repeatedly acted in clear, open, and deliberate public opposition to Brigham Young (LDS Prophet) and other members of the Twelve. She persisted on teaching polygamy as false doctrine and formally joined the RLDS church.

But she was not excommunicated.

What if they would have kicked her out of the Church?

It might have been the end of Mormonism as we know it. It's like excommunicating Mrs. Monson after Thomas Monson passes away.