Voldemort was dead: to begin with. There is no doubt whatever about that. Neville had sliced the snake like a Christmas cake. Harry sacrificed himself only to rise again and kill the last mangled bit of Voldemort’s soul. Voldemort’s body was destroyed, cremated, the ashes mingled with the angelica archangelica herb and scattered to the four winds. No one believed he would actually regenerate after his horcruxes were destroyed, but for the Wizarding World’s peace of mind, all precautions were taken. Snape himself had helped to gather the herb. Old Voldemort was as dead as a door nail.
Snape knew he was dead? Of course he did. How could it be otherwise? He had practically felt Voldemort’s death like a pulse of energy that shook the ground and rattled the remaining glass in the windows of the Shrieking Shack. Of course, he had been lying down on the ground, bleeding out of his neck, trying to unscrew the bottle of anti-venom, and cursing anyone, thing, and object that would listen, for his pain. Eventually, the cap relented and the gaping hole slowly reduced. With Voldemort presumably dead, Snape no longer saw the need to play dead. He slowly made his way out of the shack, gasping and stumbling in pain all the way. He reached a vantage point and looked on the aftermath of the war.
He didn’t feel guilty for missing the final battle; he was supposed to be dead anyway. He could leave now; no one would ever know he survived. Even as he thought on this, his head felt faint, and he realized that he forgot to take the blood replenishing potions.
“Professor Snape?!” someone gasped out to the right of him. He saw a darkening outline of impossibly fuzzy hair before the world titled sideways.
He supposed it was for the best that he was caught.