Voldemort is notorious for his fear of death. To avoid death, he is willing to kill innocent people and destroy his very soul.
Now, everyone with any sense knows that Bella is a Mary Sue, and Stephenie Meyer desperately wants to be Bella. I even personally know quite a few Twilight fans who realize and accept this. And what does Bella want? She wants to become a vampire.
Here is some commonly known knowledge about vampires (according to Meyer):
-Kill innocents for survival
-Possibly lose their souls, though this is not known for sure
We see Voldemort killing innocents for survival. He also has a complete lack of respect for animals, which ultimately contributed to his downfall. You can say, "Well, the vampires have no choice! It's people or animals!" Obviously, Bella has a choice. She can just choose not to become a vampire. Of course, Bella being Bella, she chooses vampirism.
Voldemort makes himself practically immortal by ripping his soul apart. Bella voluntarily makes herself practically immortal by losing her soul. I know, she may still have her soul, and it never said definitely whether your soul is lost or not. Still, just the possibility being there, especially at a 50/50 chance, it shows that she is willing to lose her soul to live forever. Fear of death, anyone? Just like Voldy?
The difference is that Voldemort is shown as the monster he is. He isn't strutting around pretending to be the good guy. The vampires, on the other hand, pretend to be the innocent little pretty vamps, sparkling their way past suspicion of those who do not pay attention. They are made out to be the good guys, but if you really look, you can see that this is not true.
Although, I must say, Voldemort is even better than the Cullens in one way; he fights his own battles. He fights for what he wants, even if what he wants isn't the greatest thing. The Cullens seem perfectly willing to crawl into their little hiding hole if there is even the slightest doubt that they may lose the battle. This is seen clearly in Eclipse and Breaking Dawn. They are perfectly willing to let evil triumph as long as they don't have to sacrifice even a hair. So that makes Voldemort superior to the Cullens in that sense.
Just thought I'd point this out.