According to a previous post by either OP or anon, the fallen were doing things it didn't like (which isn't hard to imagine with everything we've seen), which is why the traveler left them. Either of those two, of course, don't pull stuff out of nowhere, they have grimoire behind it. But besides that, even though this is a bit in the realm of opinion, I wouldn't call running from a sure death cowardly. Especially when you're the only thing that can beat the enemy, given the right conditions.