I tend to classify undead into two separate categories in my own campaigns. There are the Curst Undead, created by fiendish demi-deities thousands of years ago. These undead include creatures that feed off life to maintain their existence and tend to become feral when they are unable to feed for prolonged durations. Vampires, ghouls, etc.
I use a level-system for such (similar to age categories, I guess, since they are not balanced in power to class levels). They slowly grow into their abilities, and they can generally resist the hunger until level four - at which point they start receiving abilities and curses that make it impossible to resist. They can either willingly follow their downwardly spiralling path - in which they case they become Tainted in body and soul, or they can continue resisting - and go partially insane / feral as their spirit becomes trapped in a body no longer under their control. These are the vampires that die with a smile on their face as their spirit is finally freed of the horrific prison of flesh.
Prior to the fourth level their alignment is as normal, but after it is either evil (if they willingly choose to follow the corrupting path of their tainted hunger) or neutral (if they effectively become feral monsters of borderline sanity due to resisting the path and losing control of their body to their base instincts).
The second type of undead are those not curst with a hunger to feed off the living. They continue to have the same alignment they held in life, and they are typically made up of ghosts, revenants (similar to wights, but without Str drain or Spawn ability), etc. Zombies and Skeletons are treated as revenants or as animated objects (dependent on whether or not they have an Int score).
When I want to deal with horrific evil / dangerous zombies, I typically use ghouls instead. It better matches the current modern view of them, anyway (in movies such as Night of the Living Dead, in games such as Resident Evil, etc).