Might be reading too much into things but I took it to be a way of showing Mikes character. He didn't kill out of spite or vengeance, it was just business to him. Once he knew he was dying he felt no need to shoot Walt. It was a sharp contrast to what Walt had just done.
That's a fine way of looking at it.
I interpreted it as Mike simply didn't want his last act on Earth to be one of murder. He was determined to die in peace, rather than having Walt groaning and bleeding out next to him.