Going off the reservation is a common expression (despite it being politically incorrect today). It can mean to engage in disruptive activity outside normal bounds.
Originally, "going off the reservation" literally meant leaving the reservation to which one is restricted.
This article is about the phrase and its usage, including this one:
The CIA agent was not supposed to assassinate those people. When he started killing them, he went off the reservation and had to be eliminated.
Here is an article using the phrase, tying it to insubordination (but not fighting).
This Urban Dictionary definition captures it, as well (ref #2)
Going off the reservation is when certain elements such as spies or assassins either fail to complete an assignment/mission, go on an unauthorized mission, get caught and spill their guts, or go rogue. Usually resulting in the need to neutralize(kill their ass) the said element.
This introduces another term: to go rogue which means operating outside normal or desirable controls.
You can use these phrase like this:
Colonel Smith and one of his companies went off the reservation this morning, supporting the partisans in an attack on the radio station. He may have been colluding with an agent at our forward base outside the city who decided to go rogue with him.