ArikaiHart
Well, if you don't kill them all, then it's fricken hard to keep the lights from going out. So there's no one holding a gun to your head, but unless you wanna fight endless undead (or run through really, really fast and miss out on tons of exp you'll kinda need for the upcoming boss fight -and that really isn't a spoiler, because it's a dungeon, and ought to be expected)
Actually killing one restores the gauge to about 50 to 60% so there's really no need to fight them all. Not fighting them means you'll encounter endless undead enemies...yes, but it only forces you to gain more experience which is a plus, not to mention more gil from selling the hoards of loot from killing all those undead on your way to the exit. So not killing them all is a plus all in all.