I agree with Skans post. Mindset is important but you need to put it in the proper context.
When I went thru my basic training & MP school, our hammerhead drill Sgt(E-6) made us yell; "KILL" before we could eat every meal in the mess hall.
I thought this was just a stupid motivation method and didn't think much of it.
A lethal force event is fast, stressful and filled with chaos. You need to be assertive & fully prepared to use lethal force if needed but you can't walk around like RoboCop or some headcase ready to fight at a drop of a hat.
Common sense & good judgement go a long way,