Backout works by applying a changeset that's the opposite of the changeset to be backed out. That new changeset is committed to the repository, and eventually merged.
https://www.mercurial-scm.org/wiki/Backout
How can I backout without commiting the changeset? I just want the changeset reverted in a working directory.
Try hg backout --no-commit REV
This will perform the backout but leave the changes uncommited.