All throughout the battle, he has never shied away from meeting Madara's eyes. A wavering gaze is an obvious sign of weakness, and he cannot afford to even appear weak. Half, if not more, of warfare is psychological.
Instead of looking into his eyes, however, he looks through them, beyond them, into peaceful futures of unborn children.
Hashirama knows if he imagines even a splinter of anything redeemable he would be lost.
So he stabs him from behind and does not see blood-red eyes come alive.
He leaves without checking for a pulse, head bowed with the weight of duty.