My reasoning is this:
- the assailant has the belief that he will overcome the LEO, despite a self-evident disparity of force.
- the assailant is actively attempting to disable an armed officer in the performance of their duty(-ies). By the act itself, this indicates a willingness to use lethal force on the part of the assailant.
- as the assailant is actively attempting to disable an armed officer, they are demonstrating willingness to engage in a lethal-force altercation to maintain their freedom. The assailant is therefore an imminent threat to the community at large.
My actions (pretty much along with oljames03) would be to (using command voice) ask, "Officer, do you need assistance?", while checking for any responding units and/or friends of the assailant. Hearing a "YES!", and seeing no other presenting LEOs, draw, and say, "Backoffbackoffbackoff!" Any assailant not backing off is a valid target. Secure the scene,(possibly holster) and ask again if the officer needs assistance. If so, announce, "armed-friendly approaching from____!" Do your best to describe the direction you're approaching from in a way that a disoriented, wounded, individual will most likely understand, i.e., right/left/feet/from the top of your head. Remember that securing the scene means making sure that friends of the assailant won't be able/interested in finishing off either the officer or you, so further aid may just be covering a BG and calling 911. If this happens, be sure to describe yourself WELL.
In most urban areas, response should be pretty quick, but on a highway, or in more rural areas, you may be interventional fire, and first aid.