The immediate concern should be the safety of all persons at the scene, followed closely by an effort to contain the situation and keep it from escalating. If available, mental health professionals can help open a dialogue and convince the person to voluntarily seek treatment.
At the core of police work is the internal drive to catch the bad guys. How we go about getting him or her into custody is a very situational thing.
Planning for the worst also means planning for the less than worst. Have you considered how you would respond to a situation that did not require lethal force? Did you allow for the carry of a less lethal option?