Kinetic energy is direct product of velosity. Velocity and mass generate energy. With out velocity a bullet has no energy or momentum no matter how big or heavy the bullet is.

Speed, or velocity, is a measurement of distance and time. Kinetic energy is energy associated with motion.

So first you have to have mass able to be moved, then you have to move that mass, and they from that you get your kinetic energy.

But, there are a whole lot of other factors that kill as well. Such as the construction of the bullet, bullet placement, how the bullet transferes its energy.

So in regards to only speed and energy it's the transfer of energy that kills but with out the speed first and foremost that bullet is not able to transfer any energy at all.

So in a sence they both kill equally.