Could you give any government the power to euthanize innocent people and trust them to use it for only that one narrow purpose?
But what are the alternatives if people stop dying of old age? If the birth rate is even marginally higher than the very small death rate, we eventually over populate. The only way to resolve that is some kind of culling mechanism. War, starvation, and disease are what usually happens to humanity when resources get too constrained.
But now we have nukes.