Jack's worry had let his guard down, which made him an easier target than he might otherwise have been. His gun, still in its holster, was pretty much useless against the attack, and this creature, whatever he was he was stronger than Jack. And he'd had a leg up on the fight with the element of surprise.

Jack tried to fight him off but he was at the wrong angle, he couldn't get a grip on him, he couldn't tear himself away, and his mind was half on Ianto lying there on the floor and how at least if he got attacked he could keep the attack away from him. Not the most logical line of thought but then again slightly panicked as he was, it was the best Jack could come up with spur of the moment. At least he wouldn't stay dead. Not that this other knew that fact.