The hurt what sticks out to him, of course, and now Sirius is the one with guilt twisting in his gut. But Regulus takes his hand anyways, and Sirius squeezes it as though he can communicate every emotion he's feeling in the moment. His goodbye to Dumbledore is a little short. He's quite distracted.

"Reg," he says, and there's something bright in his voice-- relief, or joy, or something, even Sirius isn't certain, "I was so worried about you. C'mon, let's get you lying down."

He really looks like he needs to lie down.