After the death of their mother at the hands of Mundus' demonic hordes, Vergil took a very different path in life from his brother, Dante. Where Dante was content to live life at his own pace among humanity, seeing his mother cut down taught Vergil that might makes right, so if you want to be right, you need to have might.
Vergil renounced his humanity and sought the hidden power of his late father, Sparda, to ensure that nobody would ever be able to take anything from him ever again. Unfortunately, these pursuits of power often come at the cost of innocent lives, which places him at odds with Dante.