This sentence is taken directly from the IRS website's definition of the "estate tax": "The Estate Tax is a tax on your right to transfer property at your death."
That definition doesn't sit well with us for a couple of reasons, and frankly, neither does the tax itself. Initially, people generally assume that their property is their own, and rightfully so they earned it, maintained it and kept it (and heck, probably even paid taxes on it!). An inherent aspect of "ownership" of property is the right to divest yourself of that ownership, i.e., give it away however one sees fit. By imposing an estate tax, the government unilaterally takes some of this right away from people.
Further, every conceivable type of tax imposed on a person is the direct result of a choice that person has made. For example, if you chose to generate income, you must pay an income tax; if you choose to buy a product, you must pay a sales tax; if you choose to own property, you must pay property taxes, etc. The general rule is, if you engage in a "taxable event", you must pay the resulting tax. In stark contrast, dying is not a choice and should not be a "taxable event" it is unavoidable, inevitable and certain, and forcing a person to pay a tax because of something they can not control doesn't seem right (despite Ben Franklin's crack that "nothing is certain, except death and taxes.")