The spell being aimed the snake's way has the snake slithering to avoid it. Has Voldemort snarling and aiming a spell of his own towards Lily-- and he can't kill her, apparently can't even Crucio her while Sirius is still even a glimmer of a presence. No matter. He'll tire the girl out soon enough; perhaps immobilize her so the snake can bite her.

And if that doesn't work, he can always speed along the process of his complete takeover of Sirius Black. Once he's a proper body, after all, she's so much easier to kill.

Now to just make sure she doesn't kill the snake, instead. His laugh is cold in reply to her statement.

"You think it's going to be that simple, do you?"

There's another hex aimed her way as the snake moves again.