Angering the Gods is a Bad Idea
In a fantasy world, angering the gods tends to be a bad idea. The gods don't necessarily agree; in Greek myths you might survive angering one god if another has your back. In fantasy settings there may be some form of balance between the gods. For the sake of this answer I assume a balance between life and death
The good guys might not always win easily. The gods of life might support them, but the the gods of death are open minded on the whole "murder" thing, and are willing to hear the prayers of the villains. Unless those villains are uppity wizards who regularly steal souls from the halls of the dead to murder the living. In that case their actions would not be blessed by any of the gods.
If the very evil mage doesn't catch a hint, they might assume that their initial loss is because the zombies are less nimble, or less independent. He may come up with some clever stratagem around this weakness, but when the gods are unanimous in their displeasure, in one way or another the mage will find his comeuppance sooner rather than later.
Depending on the fantasy world, gods may not be an issue for necromancers. In general if you break the laws of magic you often have bigger problems than mortal authorities. Angry spirits, magic residue, druids, nature spirits, plagues of carrion feeders and magically enforced anti-necromancy treaties could all give necromancers a bad day.