If Antigone had told Creon that she was unaware that her action was forbidden by law, would Creon had set her free or not given her the death...

Good question.  I think that Creon is angered by Antigone's
blatant defiance of his laws and rule, but he has already made up his mind to deliver extreme
punishment to anyone who dares to defy the order.  Creon's decisions in this matter go way beyond
Antigone--Creon is still trying to establish his name and reputation as king, and he uses the law
to try to gain the respect of the people.  Even if Antigone claims that she does not know about
the order (which she would not do because it goes against her moral character), Creon would still
need to be harsh with her because from his perspective, the law rules above all and exceptions
should not be made.  He plans to use Antigone as an example in front of the people of
Thebes.