i have this theory about witches
they're perfectly sound-in-mind as far as humanity goes, they're just loving SAD. basically, they're traumatized beyond belief that they got attacked by their zombie friends/family and got infected while they were sad, which turned their nails long, destroyed all pigmentation and made their eyes glow... for some reason
they're also paranoid as forget, because, you know... zombies. they seem unaware that zombies don't attack other zombies (without bile?) though
so this means once the l4d scenario is over and all the zombies die from loving starvation or something they'll rejoin society, as incredibly paranoid slightly depressed incredibly deadly (but otherwise normal) people