Black-and-Gray Morality is a pretty good way to go—if the enemies these undead are fighting are even worse than themselves your readers will give them a lot of slack.
Alternatively, you can focus your story on these undead interacting with each other, so it becomes more of a question how they treat their own kind than others. Drowtales does this with Forgotten Realms' Dark Elves. It doesn't matter if your protagonists are Always Chaotic Evil, if all your characters are the same.
edited 12th Jul '14 6:19:02 AM by Paradisesnake
The World's Least Powerful Man
A horde of brain-eating, limb-stealing atrocities unto the laws of nature who rationalize their behavior with "Your ancestors murdered me for daring take up arms to defend my family, and oppressed my decendants for 300 years, so I get to be a dick. What's your excuse?" and an entire nation of slavers who have approximately two or three towns of decent individuals.
edited 12th Jul '14 11:04:38 PM by dvorak
Now everyone pat me on the back and tell me how clever I am!
Basically; I'm working on a Recursive Fan Fiction of Chains that focuses on an Army Of The Dead led by Prince Tanelon, come back from the dead as a Revenant Zombie, and a toymaker killed in the War of Betrayal. I'm wondering how I can make them sympathetic when they're cruel, belligerent SOB's who nearly outstrip their opponents in terms of general unacceptability.
edited 6th Sep '14 11:59:33 PM by dvorak
Now everyone pat me on the back and tell me how clever I am!