As I recall it, the issue was that Washington stated that he would hang a British officer in response to Hales hanging. A very young British officer (cant recall his name) was slated to be the one, and Washington spared him after the mans mother wrote to him personally it was something of a sensation at the time. So having relented once, he couldnt very well relent again, even for the sake of an officer who was admired by both British and Americans. Even so, Washington informed Howe that he would exchange Andre for Benedict Arnold, but only Benedict Arnold. That of course did not happen.
I’m not sure where you got the idea that Washington thought Andre was a fine man: but I don’t believe that to be the case.