The only way to kill something (quickly) is through blood loss or CNS damage. Assuming that hitting a target as small as the CNS in a fast moving, three dimensional, unpredictable, high stress environments is not going to happen reliably - then you should be shooting center torso for the purpose of blood loss. Your bullet needs to penetrate through the body (bones, muscle, etc) and do as much cutting as possible to maximize blood loss. That is it.
Now I would say, and this is just personal opinion mingled in with various facts, that a bullet needs three things to kill:
1) Mass - enough inertia to pass through the body. Very light bullets simply do not have enough mass to do this. They will transfer 100% of their energy to the much heavier body nearly instantly, resulting in all of their energy being transferred very shallowly.
2) Velocity - the faster the bullet is going, the more energy it has to pass through the body. Tissue does not have time to "move out of the way" of a faster moving bullet, resulting in more tissue damage. There is NOT that much energy in a bullet, the reason it does so much damage is that the effects of that energy are "deposited" into the body in such a short amount of time.
3) Bullet design - cutting edges and tissue damage. Hollow points have sharper edges and larger diameters, so they can do more tissue damage and cause more blood loss.
So assuming that your bullet has been designed to achieve every bit of cutting that is physically possible, the best way to maximize lethality is to make the bullet heavy enough to go through the entire body (because the more damage the bullet does, the more blood loss occurs), and with enough velocity to go through the entire body.
Any time you try to make a bullet so that it won't pass through the body (to prevent overpenetration) you have to be decreasing effectiveness in killing the target. Period. If the bullet stops halfway through the body, then something is wrong.
Just my opinion - but I want any bullets that I shoot to go completely through the BG.