The undead has a choice to either find a volunteer, specifically target creatures that are actively evil, commit an evil act to stave off its own suffering, or destroy itself and lay claim to whatever afterlife becoming an undead stole from it. There's also the option of seeking ressurection, seeing as ressurection and true-res can return the undead to life.
Choosing to victimize someone for any reason other than your continued survival or to put an end to evil actions is evil. Undead never need to do anything at all to survive. Being turned into an undead with a feeding "requirement" is ultimately being victimized unless you do it to yourself. That doesn't make victimizing someone else okay.
However, victimizing agents of evil can be neutral or even good . It goes back to preemptive striking. Let's say a bleak one (the undead that feeds on heat) chooses to feed exclusively on verified villians. He's now a force for good and his feeding is not a problem, because it serves a good purpose without being an inherently evil action (damage of all kinds are exempted from being inherently evil if they're part of a creature's natural arsenal.)