No you don't assume the existence of something, to prove its non-existence. You assume the existence of something to prove its characteristics. For instance as an argument for an atheist, you could asnwer this question:"why is the non existence of God so obvious?" You can only answer it, by assuming nature does everything itself, but what created nature? You cannot say nature, because that is logically inconsistent and contradictory.
Please take no offense, but it appears you're ignorant to a very basic principle of debate and logic.
Please review this:
Reductio ad absurdum - Wikipedia, the free encyclopedia
I will quote a relevant portion for you:
Wikipedia said:Reductio ad absurdum (Latin: "reduction to the absurd") is a form of argument in which a proposition is disproven by following its implications logically to an absurd consequence.
How do you suppose one can follow the implications of an assertion without assuming it briefly for the sake of argument?
Please, we're not trying to be difficult, but it is in fact a legitimate argument to briefly assume something in order to demonstrate why it can't be true.