Aim for the exit hole. Imagine whare your arrow will come out the deer and aim to get the most vital tissue inside the deers chest.

It all depends on the angle of the shot as to where you should aim.

Sometimes you need to aim high on the deer to get both lungs but will pass over the heart. Sometimes you get a perfectly broadside shot and can put it square in his chest and double lung him (which is best and results in a quicker kill than a heart shot). Sometimes he is quartering away enough that you need to put it through the rear of one lung and the front of the other lung.

If you take out both lungs but not the heart then the deer still has a pump forcing blood out of its vanes and arteries. If you take out the pump then the blood simply drains out. Would you rather the blood be pumped out or drain out?

However the heart shot is obviously a fatal shot as well.