Allow me to respectfully disagree with the other answers currently given. It's quite possible to do this, and I think it's a really interesting challenge.
The first thing to remember is that bad guys don't think they're bad. In the usual plot structure, where the POV follows the good protagonists, it's hard to present this fact, but your story is in the POV of the evil necromancer. So you have the opportunity to show us the inside of the evil mind, which can be really compelling. In order to make the reader believe this, try to hit the following points:
- What is the necromancer's goal? Ideally, he should have a goal which the modern, non-evil reader can relate to, even if we recoil at his methods. Is he trying to prevent something even more disastrous? Is he trying to be reunited with a lost lover or family member? For comparison, in the last several books of the Wheel of Time Rand came very close to being an outright evil villain, but the reader sticks with him because we know that he's literally responsible for saving creation.
- How did the necromancer get to this place? Can the reader identify with and sympathize with the path that brought him to this place? Avoid cliche here, as it's really easy to fall back on "he was abused as a child" or something similar that's been done innumerable times.
- What is the necromancer's personality? What if, in person, he's a really nice guy? Maybe he has a great sense of humor or an interesting hobby. Think of Spike in the early seasons of Buffy the Vampire Slayer, who was in theory as nasty as vampires can get, but engendered the viewer's sympathy by being fun-loving and irreverent.
The examples I mentioned above (Rand and Spike) all involve characters who are eventually redeemed, but you don't have to structure your story that way. You can let your necromancer just slide continually deeper into evil, and let the reader experience the conflict of seeing a character that they like and understand become more and more irredeemable.
Edit: probably the best example I can think of from recent media is actually Tony Soprano from The Sopranos, who is very clearly a bad man who gets worse over the course of the series, but who is nonetheless the protagonist. You might consider examining the way that Tony's character is presented and built for examples of how to do this effectively.