Confidence in your ability to put the arrow where it needs to be is your number one tool in helping get into "Kill mode".
If you truely love the animals you hunt, deep inside you is the reality of what you are about to do. In your heart you KNOW it is OK, but your heart also knows the responsibility you put upon yourself, just by being there, as a hunter...a predator. Part of the circle.
This is a burden we MUST accept as hunters. I believe, this burden is what we experience when in the presence of game. The excitement we feel, is actually the acceptance of our responsibilities. I believe wolves, and bears, and cougars are free of this burden.
During the heat of July, I am getting into "kill mode" through my practice. I tear my targets up with my broadheads, because the added CONFIDENCE gained through the use of broadheads all summer, helps me during October. When a shot presents itself, I KNOW I can make the shot.
This "mental hurdle" you asked about, is best dealt with on a year around basis. It is not supposed to EVER go away. Experience, and confidence are our tools to help us. But the mental hurdle is supposed to always be there.
You asked how to get into a mode where "nothing can break your focus, and all I can do is kill the animal in front of you"...I say, shoot until it is automatic, recognize what your EFFECTIVE range is(where EVERY arrow is a kill shot), and keep your shots within that range. Beyond that...enjoy the ride.
Great thread.