If a "death tax" were ever to be imposed, it likely would have to be a flat tax. In the grand scheme, your life-death situation is no more valuable than mine, so if they tax "death" it should be a flat tax.
However, what ARE taxed are estates at the time of death, a property tax; and inheritance, a tax on the privilege of being allowed to take custody and control and to assume the right to exclude others over property of one who has died.
There is no inherent right to kinship succession.
The state allows it, a form of favoritism if there ever was one, and in return charges for itself a share of the bounty it allows to go certain ways to favored persons, from those whose actual and direct control has passed. You don't drive the Chevy after you've croaked, so why, inherently, should you have any say over who does?
A power to control something tangible or intangible after your capacity to defend and use the asset, by designating who after you cease to exist can control it, is a quite artificial thing. It is a dead hand having power.
Allowing a fight for dominion after death of an owner, or forfeiture in total to the state to be sold to the highest bidder, an exchange-based surrogate for fighting, are inherently more equitable things than saying a no-longer existing person can still call shots via something as intangible in actuality, as a "will" outlasting a life.
If absolute and unwavering escheat to the state at death, and sale with proceeds going to the common good, are not inherently more equitable, at least they arguably are so. What counter arguments beyond custom can YOU offer to the contrary? Locke thought about this a bit. King Lear thought he'd figured an alternative as did Hamlet's uncle.