...
Because that can happen. We've already seen, in Turn Coat for ex, that even Thomas, who desperately wants to not be evil, is still an unexploded bomb around mortals, potentially. As Eb said, if Thomas gets Hungry enough, the demon takes over and nobody is safe until it feeds. Eb wasn't wrong about that. Thomas would have killed Molly if she hadn't had that force-field defense ...
I mean... something similar is true for anyone strong or well-trained.
I'm no John Wick, but if I seriously snapped, facing someone untrained, I could kill them in just a few seconds. Anyone with a gun & a lost temper can do it even faster. Murphy talks about it with Harry, once or twice, likening her training as aikidoka to Harry's as a wizard; the danger that they both are, the training and discipline they both need.
It's not the same thing.
Yes, anyone can snap and kill someone else. I could go nuts while driving down the road and deliberately drive into a crowd and run down some people, etc.
But in each case those are all 'ifs'. Any given person might snap and do something lethal, but in most people it's
very improbable. There's no specific condition where it's essentially 100% certain to happen. Even a person who is dying of starvation, and whose only possible source of food is to kill and eat a friend or relative or whatever, is not certain to murder to live. People can and have died rather than violate their morals or their loyalties. Not everyone will do so, but it's always a possibility, and the flip side is that it's never certain that a given person will go bad.
But
any WV is a different story. If he or she gets Hungry enough, the demon shuts down the human personality and takes over, and that WV turns into nothing more than a very, very dangerous predator optimized to feed on humans. He or she will remain in that state until the demon feeds, or the WV is destroyed.
Not
might. Not
could. It
will happen, if the demon gets sufficiently Hungry. And that's true of every WV, including Thomas.
It's the full 'silver eyes' state, and in that state, the WV doesn't care if he or she eats an enemy, a friend, a lover, whoever. Any human within strike range is potential food for the demon.
Thomas managed, one time, to break off feeding on Justine just before she was dead. But it was apparently a close-run thing even there, and it happened only
after the demon had fed on her to the point of near-death (assuming that whole incident wasn't somehow a Nemesis deception).
A WV that kills in that state might regret it afterward. I'm sure Thomas would have been wracked with horror and guilt if he had eaten Molly. But though he might weep over her grave (assuming someone didn't immediately kill him for it), Molly would be no less dead.
The thing is that that demon is always there, always waiting, always ready to go on a feeding spree if it gets Hungry enough. One could even argue, mathematically, that it's
certain that any WV that lives long enough will, sooner or later, fall into a situation where he or she loses control of their demon. It's kind of like an asteroid impact, improbable today or tomorrow or any other given day, but nearly certain over unlimited time.
WV are dangerous.