Data isn't too concerned about the snake's head; it's the paws he needs to avoid. Being torn at by claws would cause worse damage than potential puncture wounds from fangs, and the snake's venom doesn't pose any threat to him. He sees the opening just as Jaime does, and, as planned, they attack together.

Quickly, dexterously, Data darts in to grab the monstrous snake's throat. His fingers barely reach from one side of its jaw to the other, but his strength makes up for his poor grip—it struggles, but it can't escape. Feeling the creature squirm in his hand inspires a chain reaction of uncertainties, but Data knows he can't waste any time.

One slash with the knife is enough to take the serpent's head nearly off, but it's not a clean cut. It hangs by a stubborn strip of flesh, some supernatural force keeping the snake hissing and writhing even as blood starts bubbling up through its severed neck and running down over Data's fingers. He has to strike again, and it costs him a couple valuable seconds; even as he succeeds in separating the snake's head from its weakly thrashing body, one of the beast's paws swipes out at his leg again. The tip of a claw catches his pants, catches the surface of his skin—he can sense the pressure as it pulls him open.

Retaliation isn't in his nature. None of this is in his nature. The blood on his hands isn't something he's used to; it's not something he's ever seen before. He's doing this to save the city, possibly to save this world, but it's an act of violence. This being doesn't even seem sentient, and yet they're hacking it to pieces.

Instead of striking out at the chimera in self-defense, he backs away to assess the damage and consider how to deal with its third head.