... Without googling it, you do know that the simple calculation squares the velocity term, and halves the mass term, right?
Without weighing in on the merits of the always-explosively-violent kinetic energy debate, the formula therefore doesn't just halve the mass, but also halves the square of the velocity.
1/2MV^2 = 1/2M + 1/2V^2.
No, 1/2m + 1/2v^2 = 1/2(m + v^2).
The factor of 1/2 could well be applied to either variable, and is really not relevant here. Smokepole correctly pointed out that kinetic energy is proportional to the first-order of the mass, and the second-order of the velocity, meaning that velocity contributes much more prominently to kinetic energy than does the mass.
Bottom line is, kinetic energy is necessary for the bullet to do its work, but due to the myriad of variables involved in killing effectiveness and the use of kinetic energy to deform tissue, is a poor metric of ‘killing power’. A better indicator is to simply use a bullet that can penetrate to the vitals, and make sure it impacts with enough velocity to expand properly.
