After discovering the secret to immortality, I will not share it with the world’s third most powerful wizard (or so), no matter how certain I am of her loyalty to me, or of her ability to keep secrets. The worst case scenario then is having to train up a new powerful lieutenant, rather than having to kill an immortal ex-lieutenant or trying to contain the secret once it’s out.
Evil Overlord List, revised edition:
After discovering the secret to immortality, I will not share it with the world’s third most powerful wizard (or so), no matter how certain I am of her loyalty to me, or of her ability to keep secrets. The worst case scenario then is having to train up a new powerful lieutenant, rather than having to kill an immortal ex-lieutenant or trying to contain the secret once it’s out.