“Begging the Question” is a logical fallacy in which the proposition to be proved is assumed implicitly or explicitly in one of the premises.
If one argues:
All intentional acts of killing human beings are morally wrong. The death penalty is an intentional act of killing a human being, therefore the death penalty is morally wrong.
They “proving” their argument with another assumption. Morality is not factual, everyone has different morals. You cannot assert that killing is wrong without somehow proving it is, which is impossible.
It is also known as circular reasoning.