Ehh.. Interesting thought indeed, but I think Hashi was smarter than that to look into Madara's eyes. Or if he did, I think he would've broken out of Genjutsu.
Hashirama was a descendant of the Younger Son, he believed love was the answer to peace.
He knew Madara wasn't dead. He just left him there, to figure out his choice, would he come back good? Or evil with more hatred? Hashirama didn't know, he just couldn't bring himself to kill his rival at that moment, he knew he had damaged Madara beyond repair and left him in despair. See what I did there?
Summary: Hashirama spared Madara due to his feelings Madara got even more pissed that he was spared.