I think the argument is that if you're a criminal -- and thereby willing to break the law in the first place -- making gun possession illegal won't stop you from obtaining them out of your own volition. I mean, you're a criminal. Only people who follow the law will be willing to follow the new gun law.
So in your scenario, if you're looking to kill someone, you could do so today by obtaining a weapon legally. If all guns are later banned, you'd probably still be willing to get a gun illegally, since by hypothesis you're already willing to kill someone.
But this is a hypothetical scenario, so there won't be any data showing "Murders Prevented by Murderer not wanting to Break Yet Another Law."
That doesn't mean there wouldn't be less guns overall if they were banned, just that banning guns would overwhelmingly take them out of the hands of those that obey the law when stuff is banned.