His resolve is crumbling now, disintegrating into a thousand tiny pieces and slipping through his fingers like sand. The myriad faces that Regulus has shown since entering the room have thrown Remus off kilter despite his better efforts to remain focused, and he finds that confusion and uncertainty are quickly moving in to take up residence where he'd been so certain of what he had to do before.

The goading words disturb him because for some reason he hadn't expected that particular tactic (tactic? or defense mechanism?), though it does leave him sure of one thing, at the very least. He realizes with a start – and no small amount of relief – that he was never actually going to kill Regulus (could never kill Regulus), but the realization alone doesn't change anything that he's seen or heard since the attack. Sirius is hurt and they are at war and he cannot let his emotions blind him (though isn't that exactly what he has been doing?).

Then there's a weak query from behind him and Remus nearly forgets himself, halting his turn toward Sirius only because Regulus is speaking in that voice that's barely his own and it terrifies Remus.

"Stop..! Stop, what are you telling him?"

They hadn't known what to expect when Sirius awoke (if he awoke), but the chance that he'd been Imperiused was always there. And now with that dark and honey-smooth tone Regulus sounds for the world like he's issuing a command, not a reassurance, and Sirius could still be in danger and Remus doesn't want to believe the worst anymore but he can't take any chances because Sirius is hurt and they are at war and he acts without thinking:

"Stupefy!"