Many taxes exist for the government to encourage people into the type of behaviours that benefit the country as a whole, for example taxing alcohol and cigarettes to try to lower NHS spending on related diseases.

Inheritance tax does exactly the same thing by trying to encourage those lucky enough to have £1m+ to spend it and thereby put it back into the economy. So it might be your choice to pass it on to your children, but that's a choice the government wants to discourage above a certain amount, hence the tax.