Raph doesn't know why he goes back for the helmet.
With Leo missing, and Raph's mistake, and the world going down in flames, it seems like a stupid thing to do. Then again, Raph reflects, he's done a lot of stupid things lately. Besides, he had to go back for the swords. He had to.
He remembers the first time he ever put the armour on. How it made everything better and worse, because not only was it a barrier between him and the world, but a barrier between the world and him. So Raph trudges to the rooftop with the armour still weighing him down. He used to revel in how heavy it felt, like he was strong enough to carry the weight of the world on his shoulders. Raph realises he was not so much carrying it, so much as getting crushed in slow-motion.
It's raining so hard he feels like he's drowning, but it's good, it's good, because it douses the flames. Raphael leans down. Picks up the sword. Turns the helmet over in his hands. He has no idea what he's going to do with it. Throw it away? Hide it?
Give it to Splinter?
The tails of his mask flutter crimson in the wind, and he strokes the helmet with a twisted grimace.
There is something terribly symbolic about a mask beneath a mask.