He jerks back and holds his fire for now. It's possible that Zenyatta knows something he doesn't - maybe he's able to sense it or something - and so he waits.

He doesn't have to wait long, as the creature wraps its tendrils around Zenyatta and the omnic hits the ground. He's never seen Zenyatta not hovering, and it looks for all the world like he's short-circuiting, and-

Yeah, no. Wait time's over.

"You've got to be kidding me," he snarls as he draws his knife, dodges around the downed Zenyatta, and tears through the hellbeast's tendrils with the blade. There's a time and a place for pacifism and this is obviously not it.