The death tax is an inevitable point of disagreement in a presidential campaign. Donald Trump would eliminate it to promote growth. Hillary Clinton would raise it—up to 65%, while lowering the exemption for estates to $3.5 million—to promote equality. The outcomes would be as different as their intentions.

What’s less remarked upon is that estate taxes are always double taxation. Estates are built with savings that have already been taxed as income, or soon will be. Even contributions to tax-deferred retirement accounts will...