Did Jephthah sin at the moment when he sacrificed his daughter?

Suppose for the sake of this question that Jephthah really did burn his daughter, and did not dedicate her as a virgin. Now I am guessing that Jephthah sinned at the moment when he made his oath, but did he sin at the moment he burned her? Or did he not sin at that moment, because he was merely following the oath he had previously made? Should Jephthah have refused to carry out his oath to God?