Bella’s obsessive love for Edward made her revolve her life around him. She became so dependent on him. This dependence grew into selfishness. She will do everything just to be with Edward for forever. She kept on convincing Edward to make her immortal although he’s very against it. She became so self-centered and began to be obsessed with the idea of spending eternity with Edward. Even if it means becoming like him. A vampire. She broke a lot of hearts along the way in her quest to immortality. She broke Jacob’s heart, manipulated him to her advantage. She used Jacob’s love for her and made him bid her wishes. In New Moon, she was so depressed for losing Edward, and since Jacob was there with her, she thought she fell in love with him, but she left him when Edward came back.
Both of them having been an only child grew up as loners who never fancy having friends. Bella, who often travels with her mother, never had real friends and she sometimes opted to spend her time alone. She had always had the feeling of being out of place with people - even her mother, the one person she was closest to due to their opposite personalities. Voldemort, on the other hand, never had friends from the start. Even as a child, he already exhibits weird qualities as described by the orphanage worker. He knew he was different from the other children; he was intelligent and far more powerful than his peers, and had no issue with punishing those who did not do as he told them. He spends most of his time alone and doesn’t consider his subordinates as friends or the other people around him. He hated the Orphanage, which was a reminder of the poor and common start to his life; ultimately, he thought of Hogwarts as his first real home.
As both characters grew, they began to discover their true desires. Bella for her love of Edward wanted to live forever. Voldemort for his obsession of power, wanted to live forever. Both wanted to have immortality and are willing to take the necessary risks just to satisfy their desires. Their journey/quest for immortality leads them to also finding their own identity. As they embark their journey, we began to unfold their true characters and personalities.
Her lust for immortality is shown in her constant nagging of Edward to turn her into a vampire base purely on the fact that she doesn’t find herself beautiful or worthy of Edward and to spend an eternity with him. Her insecurities always manage to get in her way especially in making decisions. One of the main reasons why she would always spend time alone is because of her insecurities. No matter how many times Edward objects, she still keeps on pushing the idea of her as a vampire. She doesn’t mind what other people might say or feel with the decision she makes. In the New Moon and Eclipse, the idea of a vampire having no soul came up, Carlisle explained everything to her and yet she’s willing to sacrifice her soul just to be immortal. Her idea of love is spending an eternity with Edward. All throughout the book, Bella is depicted as an obsessed lover of Edward. One critic even states that Bella gets what she wants and discovers her worth "by giving up her identity and throwing away nearly everything in life that matters."
Voldemort, on the other hand, was a cruel, self-obsessed/selfish, maniacal, calculating psychopath who was incapable of feeling love or compassion and showed as little mercy to his followers as he did his enemies. He was very sadistic, taking considerable pleasure in the suffering of others, even his own followers. He felt no desire or need for human companionship or friendship, and was unable to comprehend the idea of any love or affection for another. He believed that that he was superior to the point that he frequently referred to himself in the third-person as "Lord Voldemort" and held an extremely high opinion of himself.
The fact that Voldemort was conceived under the love was related to his ability to not understand love; it was a symbolic way of showing that he came from a loveless union — but of course, everything would have changed if Merope had survived and raised him herself and loved him. The enchantment under which Tom Riddle fathered Voldemort is important because it shows cruelty, and there can't be many more damaging ways to enter the world than as the result of such a union.
Out of his insecurities and disgust of having a muggle father, he changed his name to “Lord Voldemort” to avoid using “Tom”, his father’s name. Disappointed and disgusted with both sides of his immediate family, Voldemort sought confirmation of his own perceived greatness through his relation to Salazar Slytherin. He desperately clung to his connections to the ancient wizard, and became obsessed with Slytherin's ideals of blood supremacy. During the near end of the final battle, Voldemort proclaimed that Hogwarts would no longer need sorting, and that Slytherin's emblem, shield and colour would suffice, thus attempting to realise his idol's dream of a Hogwarts free of Muggle-borns.
Voldemort obsessed with immortality, went further than any wizard known to history to create seven-part soul through the use of horcruxes. All of Voldemort's Horcruxes are made from objects that have extreme value, in his desire to secure his position as the greatest in history, and that only noteworthy items can live up to his standards and having the honor to house a fragment of his precious soul. As such, he had originally made it his desire to collect four items of the four founders of Hogwarts; he only found three, and gave up after failing finding another, but made Horcruxes out of other items that have sentimental value to him, if not as a priceless artifact of the Wizarding world.
The similarities between the two characters simply proved the idea that each and every literary character no matter what or how the author portrays/personifies/characterize them, still share a little of something with each other. (Although honestly, to compare the most evil and ruthless wizard of all times to a lame, clumsy and stupid human turned vampire is an insult!) :)