[There is nothing out there that could be more dangerous than what Ianto's already encountered. A new flash of fear strikes him, a terrified worry about what might happen to Spike if he comes back. He's going to hurt both of them, and Ianto is going to be just as useless as last time, too scared and helpless and stupid to do anything to get away until it's too late. He has to warn him, at least, maybe convince him to leave while he still can.]
Tyki. He was right... You've got to go, he'll come back. [Its a way of protecting him, even if it's as weak and useless as he feels.]